The Climate "Debate"
(written in the summer of 2010, with updates thereafter)

Richard Nolthenius, PhD - Program Chair of Astronomy, Cabrillo College

Purpose: This web site is intended to help the non-science student understand why essentially all climate scientists accept that global warming is real, it is human-caused, and will almost certainly be disastrous for societies world-wide. I've looked for criticisms of the scientific papers and if those criticisms are judged by good reasoning from climate scientists to be valid, I honor that. Rather than blindly look at a person's title and immediately give their paper(s) weight simply by the "argument from authority", I check that it stands the test of scientific criticism before giving it credibility. This site is my digest into more easily grasped language, visualizations and logic of the physical processes at work in climate, as too much out there tends to either be in intimidating mathematics, specialized jargon and acronyms, or conversely, oversimplified to the point it has no compelling logic. It's an ongoing project, and often includes late-breaking news and publications. Sometimes, these don't stand the test of outside critical review and in that case, I'll alter my content. It is meant for the intelligent layman, and for my Cabrillo College students especially. I debunk in as brief and compelling a logic as I can, the climate denialist claims and provide what the science says. I show what the latest research is indicating to be in our future if we continue on our current path. It is also to show what strategies are being explored to confront climate change. I provide plenty of links to longer papers in quality scientific journals, and also digests of the science from reputable sources. In some places I also provide links to YouTube videos which have animations, references, and commentary which get the points across to my intended audience perhaps much better than the journal papers referenced. Some journal papers are behind pay walls, and in this case I may link to more popular science media outlets for the gist of the results. Be prepared to see occasional outbursts of moral outrage. Engineers design components to survive worst-case scenarios, yet climate denialists will seize upon every claim that things may not be all that bad, as an excuse to justify doing nothing. To me, this is unconscionably selfish towards future life on Earth; selfish in the worst sense of the word.

I teach a Cabrillo College class Astro 7 - "Planetary Climate Science" which spends more time on the scientific principles underlying planetary atmospheres and climate. I invite you to look at the PowerPoint presentations for this course.

With those caveats, let's start...

The climate debate - Is the strong global warming of the past ~60 years due to human activities or not? But wait... is there really a "climate debate"? No. Not among climate scientists themselves. Global warming is strongly supported by the evidence to be human-caused. The idea that human-generated CO2 can change climate significantly dates back over a century. The physics of the greenhouse effect were discovered in the late 1800's, CO2's detailed infrared spectrum was determined in the 1950's and by the late 1950's there began a steady stream of scientific papers warning of human-caused climate change via our accelerating burning of fossil fuels. By 1990, data, computer modeling, and theory were solid enough to leave little doubt that human-generated fossil fuel emissions were already changing climate strongly and that it would accelerate. Here's a New York Times article - climate scientists warning of temperature rise of the same magnitude as we're still warning... check the date of the article. Since then, the widening disagreement between reality and climate models which neglect human causes has caused growing alarm and consensus that dramatic and urgent action is needed. More recently, 98% of working, publishing climate scientists support the conclusion that global warming is real (i.e. not just a wiggle in noisy climate records, or "natural variation") and caused by human activities, mainly though fossil fuel burning, according to this analysis published in the Proceedings of the National Academy of Science (Anderegg et al. 2010) . But, you say, we hear all the time on the news how controversial the issue is and it seems scientists are arguing fiercely about what is causing climate change - isn't it obvious there's a genuine debate on global warming's cause? No - the problem is that there are well-funded lobbying efforts by oil and mining corporate interests to get the public and the politicians to believe there's a big scientific controversy, as a way to stall policy changes which threaten their profit streams. While there is a very active continuing research effort to clarify key climate processes, including modeling clouds and their feedbacks, the evidence strongly indicates that while these refinements will improve the error bars, they will not change the verdict. Depending on how that goes, the business-as-usual scenario will, by the end of the century, vary from being very bad, to being truly disastrous. There is no reasonable possibility that they will exonerate humans as the cause of the rapid warming that we're seeing. And therefore, that it is up to human beings to change if we want to avoid disaster. I will show why scientists have made this conclusion, in this website.

Unfortunately, that'll mean delving into the ugly world of climate denial. If this were a resource page on purely astronomical understandings, there'd be no need to delve into politics and junk science. There's no corporate interests who feel threatened by what astronomers discover or conclude. Not so in climate research. Unfortunately, students cannot simply accept on trusting faith that what they hear in the media, or even in some unfortunate classrooms, can be taken at face value.

(note: AGW = anthropogenic global warming = global warming which is caused primarily by human activities, a useful abbreviation in what follows.


The Section Contents of this WebSite

1. The Politics of Climate

2. Debunking the Climate Denialist Claims, and their Tactics

3. The Key Evidence That Global Warming is Human-Caused, and Condensed in One Page

4. Today and Beyond: New Evidence and Consequences

5. Strategies... What is to be Done?

6. Local Resources


Growth rate of CO2 in parts per million per year, with 10 year averages included (horizontal bars). I've added the economic recessions for the past 50 years, which correlate well with drops in the rise rate of CO2. This is most dramatically seen in the "oil-shock" recessions of the Arab Oil Embargo of '74, and the first Gulf War in '91, which particularly affected the availability of oil. The correlation is obvious.

So What Can We Conclude?
We have confirmation that the observed rapidly rising CO2 levels are coming from fossil fuel burning from the known emissions rates, from the unique carbon isotope ratio found in oil and coal laid down hundreds of millions of years ago (Ghosh 2003, and for the layman here), from the observed signature of economic recessions in the CO2 rise rates (see figure at left), and other data. We know the physics of the greenhouse effect with high precision, we know the distribution of CO2 around the planet and that it is thoroughly mixed on a short time scale, and that the physics of the greenhouse effect leaves no doubt that man-made GHG (greenhouse gases) will cause warming of the troposphere and cooling of the stratosphere at levels consistent with real observations, even without complex climate models. We know that study after study has found that humans account for ~100% of the global warming over the past ~60 years (see graph at right).

Net human and natural contributions, in percent of total, to the observed global surface warming over the past 50-65 years according to Tett et al. 2000 (T00, dark blue), Meehl et al. 2004 (M04, red), Stone et al. 2007 (S07, green), Lean and Rind 2008 (LR08, purple), Huber and Knutti 2011 (HK11, light blue), and Gillett et al. 2012 (G12, orange). Note that "natural variation" contributions in most studies are actually slightly cooling the planet, so that human-caused global warming is actually slightly stronger than 100% of the warming that we observe.

We know that the denialists have provided not a single theory shown to be consistent with the observations, nor even an idea which can pretend to be a plausible explanation for what we have seen. They attack by seeding doubt, by using lies, and they attack by smearing the integrity of climate scientists and even the process of science itself. We know that hundreds of millions of dollars of corporate money is being funneled to support climate denialists and that their media and political connections have succeeded in disinforming lay people, politicians, and even teachers who should know better, playing on their psychological tendencies. We know that this is especially prevalent in the United States, and that people in Europe, Asia, and Latin America, for example, are much better aligned with the scientific consensus. As of 2009, only 36% of average Americans believe that global warming is caused by human activities, from a Pew poll. We know that there is already further warming of 1 degree Fahrenheit "in the pipeline"over the coming decades which will happen even if, against all politico-corporate inertia, we keep CO2 atmospheric levels constant, due to the fact that the Earth is not in radiative equilibrium. The atmosphere cannot transmit as much heat outward as we receive inward from the sun because greenhouse gases are accumulating too fast to accommodate to. And too, that 93% of the greenhouse heating that has already occurred has been deposited into the oceans, where it remains, and where it will prevent any cooling back down, due to well-understood thermal inertia and the decreasing ability of warmer oceans to absorb atmospheric CO2. The climate system is physically large and therefore takes long time scales to change direction. It's the proverbial Titanic. Long time scales... but not long enough. Too long to motivate politicians, who evidently think primarily of themselves and the largess from lobbyists of their corporate overlords. But too short for those who care about their children, grandchildren, and all future generations. The damage we've already initiated will require vast efforts to stop in time. The time to turn the Titanic is not when you're just a few feet from the iceberg. This makes the Titanically stupid comment of Ronald Reagan that much more an indictment of how we select our leaders: In 1979, after being told by climate scientists that climate change will be seriously affecting life by 40 years in the future (i.e. today) Reagan's response was "...Get back to me in 39" (quote is 39 minutes into this BBC documentary) .

We know that the relatively mild policy proposals so far discussed, including the older Kyoto Accords, are inadequate to prevent economically and environmentally devastating consequences. We know that even these proposals can't get any political traction, A recent study from Media Matters found that the Sunday shows on NBC, ABC, CBS and Fox spent a combined 27 minutes on the topic of Climate Change for the whole of 2013. Meet the Press was singled out as “failing to offer a single substantive mention of climate change” for the entire year. This conspiracy of silence adds to the efforts of denialists (example: the CRU email theft just before the Copenhagen Climate Summit , and see the denialism of the Republican candidates of the 2010 election). The Copenhagen Climate Summit, thanks in substantial part to the efforts of climate denialists, was a dismal failure. Even the one "accord" agreed upon, that we must confine total global warming to no more than 2 degrees Celsius, is derided by scientists as far too much ("2 C is the boundary between dangerous and extremely dangerous climate change" - A. Bows; Tyndall Center for Climate Change Research), and yet at current emission rates, by 2028 we will have already used up our entire allotment (~565 billion tons of CO2 emissions), which can have some hope of keeping us under 2 C (and this assumes the old and overly conservative IPCC AR4 climate models which neglect important positive climate feedbacks). This represents only 1/5 of proven oil reserves. Said another way, if we continue on "business as usual" till 2028 and then STOP ALL EMISSIONS PERMANENTLY, we will have committed to +2 C of warming (at minimum) and that warming would be permanent. Realize that +2 C is more than 1/3 of the warming that happened emerging completely out of the depths of the last great Ice Age. In the face of this, oil and coal companies continue to spend hundreds of millions of dollars to discover and plan to exploit additional fossil carbon which, if burned, will devastate our future for millennia to come.

We need immediate and severe reductions in CO2 emission, followed by a reversal of atmospheric concentrations of CO2, a fact which is in such strong conflict with the desires of the Third World and Asia to adopt the conspicuous consumerist lifestyle they envy from the West, that it may well take a whole series of catastrophic events spaced closely enough that they do not exceed our shrinking attention span - events unequivocally tied to global warming - before civilization will consider taking the drastic emergency steps required to prevent the worst consequences. Just as serious as climate change, is the effect human-generated CO2 is inflicting on ocean ecosystems due to acidification (links here, here, and references therein).

Maybe this is a good place to repeat something I have, for 27 years now, told all my Astro lecture classes at one point or another - Nolthenius' First Law: People Learn the Hard Way. Most (a voting majority certainly) are unable to make real and compelling enough within their own minds what the difficult truths are, and to support the necessary action. I believe they will eventually commit to serious action when they see the world seriously going wrong immediately around them (not just headlines happening to someone else) - but by then it's far too late. That is the inexorable physics of climate time scales, and the very unfortunate time scale for climate forcing and response, as we'll see.

The Larger Problem - Living Beyond the Carrying Capacity of The Earth
Burning through, in a ~ hundred years, the fossilized photosynthetic solar energy of ~50 million years from the Carboniferous Era - is symptom of a larger

Ecological footprint for the United States. While the ability of U.S. land to support population is dropping, the ecological footprint more or less kept pace downward due to the "green revolution", but in the 21st Century we're falling badly behind further.


Ecological footprint for the World, from the U.N. It takes 1.7 Earths (Earths with today's ability to support life) to sustainably support today's population. Business-as-usual guarantees exploitation at maximum speed, before your competitors take it first.

problem. We on Earth have been living far beyond the dwindling ability of the planet to sustainably support us. In 2007, studies show that humans used 50% more ecological resources than the Earth could replace in that year, and that gap has been widening at an accelerating rate (see 2012 report). Humans and our domesticated livestock have gone from being 0.1% of the biomass of all land vertebrates 10,000 years ago to now being 97% today. Wild vertebrates - from lizards on up to elephants - now make up less than 3% by mass of all land vertebrates. Oil is not renewable, and neither is topsoil (we're losing 1% of the Earth's topsoil every year, due to standard agriculture practices. Topsoil is irreplaceable on anything but geologic time scales). World population will reach 9.5 billion by mid-century by current projections. Our planet can, with current technology, support this many people sustainably at a standard of living only equivalent to today's Ethiopia, according to a number of studies at Stanford University (and here). Ethiopia has one of the harshest standards of living on Earth, a place of widespread grinding poverty. We're like the spendthrift with a credit card. The ultimate end - bankruptcy. This is what awaits our children. This has been known for many years, in fact for centuries if you consider John Stuart Mill. We, as a world, are in denial. Already, rather than real growth, we are increasingly getting only the illusion of growth. Governments worldwide feel compelled to destroy their balance sheets by increasingly buying their own debt in order to force low interest rates. This forces savings out of safety and into stock markets of questionable integrity. This, because true growth is becoming impossible. Yet growth is urged from the pulpits of nearly every politician and economist as the savior for mankind. In fact, it will kill us as surely as does the uncontrolled growth called cancer. According to astronomers, we have very roughly 100 million years left of livability on the only planet in the Galaxy that we have found which can support complex life. We have approximately a decade, maybe two of living at anything like the way we have been, before disasters arrive in earnest. Right wing ideologues think we can simply hi-tech our way out of this. We will not. Jevon's Paradox (or rather, variants) continue to rule, and that is fatal on a finite planet. Virtually all of the arable land on Earth is already growing crops, nearly all large fish in the ocean are already gone, and the "green revolution", which did indeed delay the "Population Bomb", already happened and is now falling behind (see graph. However, new and heroic measures to raise crop productivity do continue). Temperatures and CO2 levels are just two of the "hockey sticks" that are happening now. This site (2009) collects a large number of studies on the unsustainability of our paradigm and the urgent need for growth to give way to contraction and then to a stable zero-growth world if we and our fellow species hope to have a livable long term future.

I'm haunted by the results of the classic "delayed gratification" studies (and here) of children, which show that the willingness to delay gratification for ultimately larger rewards in 4-year-olds is predictive of later measures of intelligence and success in life. We, as a planet, behave like the immediate gratification 4-year-olds in these studies, preferring to eat through our seed corn now rather than clearly acknowledge what that means for our future. What's interesting about the studies is that the choice is so easily grasped by all (one candy now, or two candies if you wait a bit), that it is not a test of the ability to understand what is being asked, it really is a test of the willingness to pause and make real in one's mind what the future will hold, vs working to avoid that awareness.

Meanwhile, as the American public and politicians appear unwilling or unable to see through the fog financed by the oil and mining industry and face up to human responsibility for global warming, the rest of the world moves on. China, only recently having elevated itself out of Third World status, is already leading the U.S. in most measures of clean energy technology development and use, and by a significant margin. China is hardly a model world citizen, but they are not indulging in the self-deception that we are, here in the U.S. That said, China sees the U.S. making no efforts towards facing climate change, and they have a deep collective feeling of insecurity which manifests as an insatiable desire to become the #1 world power once again. So, clean technologies or not, net/net, China continues to grow its CO2 emissions, now at twice the rate that the U.S. is, and their coal plants in particular are pushing the climate past tipping points (Simons 2012) and towards environmental disaster (However, in 2014 there is some indication China's coal use may be leveling off, as officials become more worried about revolt from the Chinese populace reacting to the incredible pollution they endure. China and the U.S. are the #1 and #2 emitters of carbon, accounting for 40% of global emission rates as of 2013.

All of human civilization, beginning with paleolithic man emerging from the last Ice Age, has taken place in this time frame, and enabled by a stable climate with constant sea level, that allows consistent growing zones and building of civilization infrastructure. Now look at the man-made carbon-induced climate change that has started. Taken from Marcott et al. (2013), and described for the layman here

A Few Degrees Warmer - So What?
You may think - "What's the big deal? Heck, I don't even know if I can tell when it's a couple of degrees warmer. Temperatures go up and down more than that every day of every year. Maybe I'll have to crank up my air conditioner a notch during summer, maybe we'll have to say good bye to a few species. It's not a disaster. What this thinking fails to grasp is two things. First, that this change in temperature is held consistently, not oscillating as in the daily or seasonal cycle with no net change in the average. A consistently held temperature rise, even when small, has very strong consequences. Second, that the world is adapted to a global climate which is disappearing far too rapidly - species cannot adapt when change happens this quickly. Species adapt when changes happen slowly over thousands of years or longer. When change of this magnitude happens in decades - species instead go extinct. A few degrees, held permanently, will melt all the ice of the poles, will cause mass die-offs of present forests and crops, will melt Arctic lakes and tundra releasing large reservoirs of ancient methane. Methane from the melting permafrost is now calculated to add fully another 30% or more to greenhouse forcing, beyond what has already been assumed in the IPCC AR4 of 2007 (UN report 2012). A warming ocean has the outside possibility (but still poorly understood) of destabilizing large continental shelf methane hydrate deposits in a positive feedback loop, rapidly escalating warming still further. A new study of past Siberian climate finds that Arctic permafrost melting all the way down to the level of underground caves, begins at a temperature of only 1.5 C above pre-industrial levels (Vaks et al. 2013 and Science News summary here). The Vaks study does not include a newly identified positive feedback - the record wildfires that are now beginning in the Boreal forests, with their killing of the insulating vegetation and exposure of the permafrost to blackened ground warmed by the sun (Kelly et al. 2013 in PNAS, and good digest for the lay person here). We are already halfway there (+0.88 C), and the other half is guaranteed "in the pipeline" even if CO2 levels in the atmosphere are held constant into the future. The release of Arctic methane was not included in the IPCC AR4 projections, due to the large uncertainties at that time of how it would evolve. Clearly that underestimates the contribution to global warming. However, the recent Siberian methane explosions and ocean release is still very tiny compared to the amounts needed to launch the "methane apocalypse" that some bloggers fear.


As sea levels rise, countries will lose their most valuable and populated property - coastal property. You may say - there'll ALWAYS be coastlines, so what's the big deal? But as sea level rise accelerates, we enter a new paradigm which will last thousands of years, of shoreline positions constantly migrating inland without pause, and therefore unable to hold economic value. Ponder the slow-motion disaster that is Florida. A recent paper by geophysicists (Raymo and Mitrovica 2012 in the journal Nature) indicates that, based on the observed sea level rise during a comparable temperature rise such as we are seeing in the 21st century (400,000 years ago during interglacial MIS 11), we saw a sea level rise of 31 feet above today's level. In 2014, Cryosat-2 satellite data confirmed that the West Antarctic Ice Sheet crossed the critical tipping point and its disintegration is now unstoppable over the coming century or two. The same Cryosat-2 satellite probed the under-ice topography of Greenland's coastal glacial grounding interfaces and find the same issue - and that the melting of Greenland will proceed much faster than we had thought even in the most recent IPCC AR5 report. That will devastate most of the Earth's great cities. 80 of the 100 largest cities in the world will be mostly or completely underwater before stabilization of coastlines is possible. Countries like Pakistan (a nuclear power, and not a very stable one) and Bangladesh will lose a high fraction of their agricultural land, and a significant part of their entire country, with salt water intrusion ruining much of even the dry land. Asia is not only the most populous place on Earth, but also the most vulnerable to rising sea level. Half of Pakistan and Bangladesh's critical rice growing land will be lost. Many island countries will simply disappear beneath the waves.

Crops will fail as rain patterns change. Wildfires will become larger and more widespread; yet another positive feedback further adding CO2 to the atmosphere. Here in California, droughts are predicted to become more common and more severe. And new studies predict "mega-droughts" later this century, on our current path (Cook et al. 2015). This is confirmed by the observations that the Hadley cell boundary (which now borders California's southern edge), marking the latitudes of descending drying air that cause the Earth's great deserts, has already moved northward by ~5 degrees of latitude (about 350 miles) since 1979, corresponding to the period of rapid CO2-induced climate change (Johanson and Fu 2009), and this continues. Drought in the mid latitudes is and will continue to fuel more fires and the ash from these fires is finding its way to the ice caps, especially worrying is wildfire ash and coal plant pollution dirtying Greenland's pristine ice, lowering its reflectivity, accelerating further the melting of the ice. Ice also becomes darker when it partially melts, affecting the geometry of the snowflakes.

Rapidly rising CO2 will continue to be absorbed into the ocean much faster than it can be taken out by natural processes, raising its acidity and ruining the ability for aragonite calcium carbonate to form, which is the basis for a vast array of sea life which depend on this chemistry. The pH of the oceans have changed more in the last 150 years than in the previous several million years. The effects on all aragonite calcareous ocean life forms is serious. This includes corals which form the habitat for 1/4 of all marine fish species at some point in their life cycles. Already we are seeing mass die-offs of coral reefs. Half of one of the world's great heritage sites - Australia's Great Barrier Reef - is already gone, due mostly to weakened and bleached coral combined with increasingly severe storms. Prior to the 1980's coral bleaching of the Great Barrier Reef was not seen. Now, it is common, due to rising ocean temperatures and acidity. Corals can only survive in a bleached state for a few weeks, Even using the overly conservative 2007 IPCC scenarios, by mid-century the oceans will be too acidic for the survival of coral reefs (Lang 2007), and they will disappear (Silverman et al. 2009). In CO2 conditions corresponding to the end of the century with "business as usual", experiments at the Great Barrier Reef show corals will be gone (De'ath et al. 2013). At higher acidifications, more carbonate-based species will perish. Already, at the beginning of this cascade of global warming effects, rising ocean temperatures (93% of greenhouse global warming so far has been absorbed by the oceans) have reduced marine phytoplankton substantially over the past century. Understand that 45-70% of the oxygen in the Earth's atmosphere (Fenicle 1983, and here) is produced by marine phytoplankton. Here is a good introduction to anthropogenic CO2 and the consequences of ocean acidification. Here is an introductory video program on ocean acidification designed for the general public. There are already serious pH-induced reproductive failures among shellfish happening now. A key phytoplankton are pteropods, and they are already dissolving off the West Coast of the U.S., decades before this was expected to happen. The pH in the Puget Sound of Washington, uncorrelated with any local pH drivers, has plunged (acidified) from 8.3 to 7.7 in just the past decade (Wootten 2013). Now, in 2013, scientists find that ocean acidification increases greenhouse warming (Six et al. 2013), via the production of sulfate compounds which go back into the atmosphere - another amplifying climate feedback not included in IPCC climate models. Freshwater systems face a bleak future as well. The Great Lakes are predicted to degenerate into a soup of deoxygenated water and surface scum, as indeed is already beginning in the most fish-rich (historically) and southern-most of the lakes; Lake Erie (Michalak et al. 2013).

Climate change is rapidly destabilizing our ability to provide food for a still growing population. Already, we have seen revolutions whose initiating cause was famine. Climate change is implicated in recent studies with the famine-caused rise of Isis and radicalism in Iraq and the middle East (Kelly et al. 2015, and summarized here). The drought which led to this political disaster is predicted to get significantly worse in the next decades...The Middle East has already seen decades of drying, which is expected to intensify for the next 20 years and beyond as a result of climate change. In less than 20 years, over a quarter million Indian farmers have committed suicide as their wells have dried up and the water table has receded down beyond 1,000 ft underground and they cannot afford to drill that deep. Many committed suicide by drinking the pesticides needed to keep their GMO (genetically modified) crops alive (Weisman 2013). How are we going to produce as much food in the next 50 years as we have already produced during ALL of human history, as the United Nations study group has recently determined we must? How can we do that while the very climate that crops must grow in, ruins soils, depletes ground and surface water, and there is no more arable land to cultivate? Crop yields drop 10% for every degree Celsius of global temperature rise. Here's a good meta-study of the scientific papers on the link between social violence and climate change published in 2013. Dismissive reassurances from Exxon's CEO that "we'll adapt to that " conveniently ignore that all of human history has occurred during a period of very stable climate and unchanging sea level, and worse still, unlike past climate changes (CO2 certainly has been higher in the distant pre-human past, for example), the anthropogenic-driven climate change of the present is happening 10 times faster than any previous climate shift of the last 65 million years, according to Stanford researchers (Diffenbach et al. 2013). New studies (2012) by integrative biologists provide evidence we are rapidly heading towards a world in which it will be difficult for us to live (versus just sending off to extinction many of our fellow creatures whom we deem not sufficiently immediately financially useful to us). Exactly how sensitive is climate to a doubling of CO2 levels? Pagani et al. (2006) argue that to explain the Paleocene-Eocene Thermal Maximum may require a much higher sensitivity of global temperatures to a CO2 doubling (including all feedbacks) than has been conventionally assumed. This argues that positive feedbacks (clouds most likely) are more powerful than the base case assumes. This conclusion is also consistent with the work of Fasullo et al. (2012), who finds that it is the most "alarming" climate models which do the best job of predicting what we have already seen. (see an interview with Fasullo on this work here).

Scientific and Societal Uncertainties
The largest scientific uncertainties are likely in cloud feedbacks. At the currently small temperature changes already seen, cloud feedback appears to be a small net positive (i.e. clouds amplify the GHG induced warming), but we can't rule out that it is in fact a small net negative, although there is no evidence nor theoretical backing that they are or should be negative. For larger temperature rises such as are expected we have not enough theoretical or computational ability to say with confidence how clouds will respond. Increased cirrus clouds such as we might expect from stronger, taller convective storms driven by the warmer ocean would, in and of themselves, be a positive feedback, but there may be other effects. Aerosol feedbacks are also not as well understood as needed. Sulfate aerosols act to cool climate, but soot aerosols act the opposite. Aerosols also act as cloud condensation nuclei. The greatest uncertainty in aerosol calculations is how much humans will contribute pollution aerosols into the future. Global climate models usually assume no cloud feedbacks. Aerosol assumptions vary. Is it possible that in the hotter climate regime we are entering, a strong negative cloud feedback could develop and significantly reduce eventual heating? Not impossible to rule out but I've yet to see a paper which has survived critical scrutiny by climatologists which predicts this. Recent studies in fact say the opposite (Sherwood et al. 2013 in Nature). And we have certainly seen extreme climates in paleo climate data. Confronted with this, we could nevertheless cross our fingers and hope. Is it possible that despite warming, that Greenland will acquire enough additional ice at its highest elevations through increasingly humid air to offset melting at the coast and moderate sea level rise? Can't rule this out with 100% confidence, but it is certainly inconsistent with the best and also the most current data, and quite inconsistent with paleo data from past interglacial climate periods.

Equilibrium response of the global temperature as a function of CO2 concentrations, based on three different approaches. a) from the PALEOSENS workshop, using data from the late Pleistocene of the past 800 kyr; b) Using data of the past 20 Myr from RW_11; c) Based on JH_12 using similar data of the past 800 kyr as in a); and d) Combination of all three approaches. Plotted areas include uncertainty estimates of one standard deviation from PALEOSENS.

ECS with "fast" responses only, is 2.2-4.8 C. Millennium and longer time scale feedbacks raise this to ECS=~7 C . This confirms earlier work of Hansen et al. 2008 who find fast+slow ECS is +6 C

Paleoclimate-based equilibrium climate sensitivity estimates from a range of geologic eras.  Adapted from PALEOSENS (2012) and here .

Range is +2 to +5 Celsius

Here's another case in point about apparent uncertainties: Climate sensitivity to rising CO2 is not as accurately known as we would like. A recent paper by Otto et al. 2013 (pdf here) finds a low equilibrium climate sensitivity (ECS) of +2 C, vs. the 3-4 C favored in the IPCC AR4 based in part on paleoclimate data. While initially hopeful upon encountering the Otto et al. paper, I became much less so after a careful read through. The analysis simply takes data from the late 19th century up to now and extrapolates via climate models to get to a full doubling of pre-industrial CO2 atmospheric levels. But these climate models do not incorporate key observational evidence and positive climate feedbacks . By using these inadequate models, Otto gets warming which follows an unchanging trend with no amplification. It finds the next 40% (of rise in CO2) will be like the first 40% which got us to the present day, so that the first 0.9C we currently have experienced halfway, is followed by another 0.9C when we go the next 40% for a resulting full doubling (1.4x1.4=2). Yet consider that it has taken up until now to slowly melt through most of the permanent ice of the Arctic Ocean. In 2011, we had already lost 75% of the Arctic Ocean ice volume present in the mid '80's (Overland et al. 2014). Ice, whether thick or thin, is still white and reflective. When the summer ice is gone - a time which is rapidly arriving - there's a dramatic shift in reflectivity which is only now beginning to be felt. A new paper by Eisenman (2014) uses satellite direct measurements to show that our prior model-based figures for Arctic Ocean solar absorption were a factor of ~3 too optimistic. The albedo of the Arctic has dropped enough to add an additional 25% to global heating on top of the greenhouse gas heating due to added CO2 since 1979. High temperatures in Siberia are already causing serious melting of the permafrost. The darkening Arctic is accelerating the thaw of the permafrost and the 21st century steepening in atmospheric methane levels may already be reflecting this (although we are not 100% certain this accounts for the change in slope of methane rise rate seen in the graph above). This is a positive feedback which has no counterpart in the dataset used by Otto et al.. Paleoclimate observational data does, of course, include all feedback effects. Studies using paleoclimate data find an ECS which consistently falls in the +2 to +5C range, with 3 C the most likely value (at left: right panel graph). Lewis also found a low CO2 climate sensitivity using only recent data (not so good) and Bayesian statistics (good), but his definition is not consistently defined and in fact looks to be a transient climate sensitivity rather than equilibrium climate sensitivity, which makes a large difference. He also apparently uses an outlier value for aerosol indirect effects which sends his ECS estimate lower - a fact which some climatologists are surprised got past the referee. Lewis also misrepresents the Aldrin et al. 2012 review article as showing agreement with his value, when it does not. See discussion of these last points here

Even if the Arctic methane and Arctic ice loss albedo effects somehow were not to raise equilibrium climate sensitivity above the Otto et al. estimate, a "business as usual" scenario or anything close to it, will result in global temperatures rising more than 2 degrees Celsius above the pre-industrial temperature well before the end of this century, and the fossil fuel companies are determined and confident they will do just that, as evidenced by their investing hundreds of millions of dollars in exploring for even more carbon fuel sources to exploit, beyond those already identified - and the existing known reserves already are 5 times the amount that can be injected into the atmosphere and have any hope of confining temperature rise to +2 C above pre-industrial levels (even the International Energy Agency is now acknowledging this).

Human Response
What will be the response of civilization as rapid climate and ecological change accelerate in coming years? Will we rise to the occasion and work together? A 2013 study looking at global data over the entire span of human civilization, find a significant correlation between hot weather and violence, rioting, and civil war (in Aug 1, 2013 edition of Nature, summarized here). World wars, in fact, have started over much less. Fighting over desires or "status" is one thing.... perhaps tempers can be calmed.. But fighting over basic food, water, and the very existence or habitability of the land you live on, is quite another. The +6 C global temperature rise which is now a serious possibility in a "business as usual" scenario for the end of the 21st century, is larger than the +5C global temperature difference between the depths of the last great Ice Age and the current warm interglacial, before human-caused global warming. This is happening in a mere century or so, rather than over thousands or millions of years, as in past climate changes. It is not so much the total amount of heating, but the speed of the change that is so unprecedented and so damaging to civilization and to all ecosystems.

These changes are predicted to persist for 10's to 100's of thousands of years (Eby et al. 2009, Zeebe et al. 2013). The higher global average temperatures we cause now, will remain high. They will not go back down - even if we stop all carbon emissions today - temperatures will not drop (Solomon et al. 2009, Gillette et al. 2011, and Mathews and Weaver 2010). Held high in this way, all ice except part of East Antarctica perhaps, will disappear. In a relentless "business as usual" scenario, within just 3 centuries, half of Earth's current population is living on land which may become uninhabitable due to extreme heat (McMichael et al. 2010 in Proceedings of the National Academy of Sciences, behind a pay wall, but a good summary is here).

Pause, and let that sink in. A few degrees will be devastating to human civilization and most of the species of life on Earth. It is worth ANY economic cost we must pay to stop it. Now. We owe this to our children and their children, and we owe it to the other life forms on this rare planet that gave birth to us.

A Final Comment
The first warnings of anthropogenic CO2-induced global warming date back fully a hundred years, to the work of Arrhenius. The evidence for human-caused global warming was strong enough in the late 1980's to be "settled" for strong policy changes. Denialism and fossil fuel corporate lobbying have combined with a science-ignorant American public (and alarmingly poor public education) to stonewall the hard policy needed to drastically reduce carbon emissions and save what we can. Denialists continue to lie to the American people, trying to convince them that strong action is premature, that more science is needed, that it's too costly to our lifestyle to think about significant reductions in CO2 emissions until the "science is settled" and worse - that climate scientists are liars, group-think'ers akin to religious zealots, and conspirators (and yes, I've heard these exact words used, to classrooms of students at Cabrillo College no less).
Consider this cautionary tale: Merrill Lynch in 2008 used a few million dollars in extra bonuses to motivate its own brokers to buy billions of dollars of Merrill Lynch-created CDO's (collateralized mortgage debt obligations) which the wider market judged to be of both very low value and very high risk. They did this just to sell these CDO's to gullible investors. This essentially was committing suicide - just a few years ago, Merrill Lynch, through its own short term greed for a few million dollars, destroyed itself - a 100 year old financial institution once worth tens of billions of dollars. In the same way, the oil and mining corporations appear just as willing to do almost anything to keep just one more quarter of big profits coming in, even if it means long term disaster for not only our children but for all future generations. Climate scientists are receiving threats of bodily and other harm. There is the prospect of McCarthy Era -style inquisitions of climate scientists by the Republican-dominated House of Representatives (and here). The Arctic Ocean is projected to be only a couple of dozen years away from losing the last of its permanent ice, with dire consequences for the albedo of the Earth and further warming. We are in the midst of the 6th great Mass Extinction since life began on Earth, with the current extinction rate 1000 times faster than the background rate. This mass extinction is caused by humans - the Anthropocene Extinction, with approximately half of all species of life on Earth predicted to go extinct before the end of the 21st century - half, or more, of all species of life on Earth. Given the overly conservative estimates on the loss of polar ice in the (2007) IPCC AR4 and the much faster environmental damage which observations are showing (see bottom of this page), a growing number in the climate science community are convinced it is already too late. That the tipping points towards a catastrophic climate future have already been passed. The oceans are already in rapid decline, and the plastics we've dumped into the oceans may kill most sea life over the coming century, disrupting endocrine biology. Atmospheric methane levels most likely from the thawing Arctic are rising sharply over the past decade... what a blindly stupid and tragic legacy we are leaving to any future generations which survive us. Climate scientists are, privately, angry and appalled at the reaction to their work. Biologists whom I know personally are so bitter that they have expressed the sentiment that we deserve the fate which we ourselves are going to suffer - very likely massive loss of human life. Future generations will look back on us with contempt. To stumble into a tragedy is one thing - but this tragedy we have known was in the making for decades, while policy makers, corporate heads, and most unforgivable of all - even some educators - decided their own petty agendas were more important than communicating the weight of evidence for what is happening, what is coming, and taking action.


From the IPCC AR4 document, 2007. More recent studies now include factors previously ignored, and show these projections are almost certainly significant underestimates of the severity of warming in the future, as Arctic Ocean melting and albedo effects are far ahead of schedule. See a more modern projection from the Congressional Budget Office below.

From Mathews & Weaver (2010) with explanation here. The red curve is climate change if all greenhouse gases are kept at 2010 values - warming continues for at least 200 years (essentially the purple curve from the IPCC graph at left). The blue curve is if we stop ALL GHG emissions in 2010 - the result is that we only keep temperatures constant - they do NOT fall. This is a vital point which seems unappreciated in the popular press. The naive notion that if you just stop doing "bad things", then the Earth will forgive us, and heal - is false. We need to REVERSE (if its possible) what we have done - immediately.


The Congressional Budget Office's chart of projected climate change for the 21st Century, based on Sokolov et al. (2009) "business as usual" emissions scenario.


Update Dec 17, 2012

Significant parts of the current draft of the new IPCC AR5 (assessment report #5 since the IPCC was created over 20 years ago) have just appeared ahead of full publication anticipated in 2014. They confirm the key points of what I've presented on this web site (no surprise). Yet climate denialists continue with their standard behavior - ignorance of basic science, shrill slander against scientists, and ideology-driven anti-science spin. See the latest IPCC graphs and the nonsense from denialist bloggers here, and the response of the IPCC author/scientist here. Still, the leaked draft of AR5 remains on average more conservative than is the actual science of the past 6 years since the previous IPCC AR4. This is, after all, a report that gets science from the scientists, but every word gets tinkered with later by the politicians; since it must get the unanimous approval of the political leaderships as well, and there are 3 times as many politicians as there are scientists, as as many "observers" who are not scientists. The IPCC is a United Nations sponsored entity, and the largest carbon emitters are also the countries with the largest contingent of IPCC scientists. According to Stanford climatologist and IPCC AR4 key contributor Stephen Schneider, China's veto power significantly played down some aspects of the AR4 report. Nevertheless, I continue to give the IPCC credit in persevering with the science they DO manage to get past all the desks along the way to publication.

Update October, 2013

The IPCC AR5 is out. Here is the 29 page summary for policy-makers. It is disappointing that oil-sponsored interests continue to block IPCC policy statements reflecting the most scientifically representative conclusions and projections, and instead underplay the dangers of human-caused CO2. According to Guardian author Suzanne Goldenberg “Nearly 500 people must sign off on the exact wording of the summary, including the 66 expert authors [leading scientists in multiple disciplines], 271 officials from 115 countries, and 57 observers.” Here's more from the IPCC scientists on what the process is for getting the document from science, past the politicians, to the public , and how the world's major oil producers succeeded in blocking key risk studies and data from being published in the last IPCC AR4. The IPCC AR5 document, in the policy summary statement (the one the politicians and governments are most concerned with, knowing only the wonks will pay attention to the deeper scientific details) follows the AR4 in making the muddled statement "95% probability that global warming is human caused". While this is up from 90% in the AR4, the statement continues to make no sense. Climate scientists will tell you there is ZERO uncertainty that CO2 has risen to the levels the instruments say, and that there is ZERO uncertainty that this WILL warm Earth's climate, and that no other cause has passed any test whatsoever for being a possible competing cause. A better statement would have said "Global warming has been X% caused by humans, plus or minus Y% ", but politicians, and that tiny fraction of oil-company supported scientists who got to be part of the IPCC, refuse to sign off on such statements. Read the actual scientific papers and you'll find no backing to this notion of even just a 5% chance it's all "natural variation" and human-generated CO2 doesn't influence global temperatures in a major way.

There is another more subtle reason for the too-mild tone of the IPCC policy statements. It is psychological and has to do with the culture of scientists in this new and unanticipated (unanticipated when they were students deciding their career path!) environment of political retribution, slander, corporate-sponsored threats to life and career. Fear... scientists are human, after all. But these climate scientists, many being the senior people representing the IPCC, started their careers when scientists were not subjected to the brutal harassment, intimidation, and slandering of today. They're psychologically unprepared for such treatment. And so, this too leans them to couch their IPCC policy statement language too much towards the non-alarmist side, despite what their actual scientific journal publications say. These aspects are discussed in Steven Lewandowsky's AGU Chapman conference talk on climate change and the media. The tragic result - only 12% of the public even realize there is a strong scientific consensus of the reality of human-caused global warming (Yale study 2014). This should sober those here in the U.S. who like to think we're the most educated in the world.


Other Resources

-- See the powerpoints and other web material I have assembled for my Cabrillo College course "Astronomy 7 - Planetary Climate Science", which has a great deal of material on current climate change on Earth

--Here's a good set of video programs covering in concentrated form the scientific case for human-caused global warming and a close look at the attempts to discredit human-caused global warming, from a prominent science writer.

--A set of videos from the National Academy of Science, for the layman, on the evidence for human-caused global warming

--Earth: The Operator's Manual (PBS program hosted by Professor Richard Alley)

--A UCAR module for the scientifically minded layman and weather people, on climate modeling

-- Geochemist and climate specialist Prof. David Archer's Univ. of Chicago course Phy Sci 134: "Global Warming" in a series of mp4 lectures.
and below as a series of video lectures for the non-science major. Note, these had been on YouTube but have been removed, and the higher resolution versions are freely available at the University Chicago link provided. I would suggest downloading them and then using your favorite mp4 player to watch them...

0 - Global Warming in Geologic Time (1hr 10min)
1 - Intro (11min)
2.2 - Heat and Light (50min)
2.3 - Blackbody Radiation and Quantum Mechanics (44min)
3 - Our First Climate Model (46min)
3.2 - The Greenhouse Effect (43min)
4.1 - What Makes a Greenhouse Gas? (45min)
4.2 - Greenhouse Gases in the Atmosphere (45min)
5.1 - What Holds the Atmosphere Up? (51min)
5.2 - Why It's Colder Aloft (45min)
6 - Wind, Currents, and Heat (50min)
7 - Ice and Water Vapor Feedbacks (35min)
7.2 - Clouds (48min)
8.1 - The Weathering CO2 Thermostat (37min)
8.2 - The Lungs of the Carbon Cycle (47min)
9.1 - The Battery of the Biosphere (43min)
9.2 - Coal and Oil (49min)
9.3 - Oil and Methane (44min)
10.1 - The Carbon Cycle Today (34min)
10.2 - The Long Thaw (41min)
11.1 - The Smoking Gun (46min)
11.2 - The Present in the Bosom of the Past (44min)
12 - Six Degrees (46min)
13 - Hot, Flat, and Crowded (13min)

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