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Mann reports: "Although the earth has experienced exceptional warming over the past century, to estimate how much more will occur we need to know how temperature will respond to the ongoing human-caused rise in atmospheric greenhouse gases, primarily carbon dioxide."

File photo, ice melting in the Arctic. (photo: file)
File photo, ice melting in the Arctic. (photo: file)

Earth Will Cross the Climate Danger Threshold by 2036

By Michael E. Mann, Scientific American

20 March 14


The rate of global temperature rise may have hit a plateau, but a climate crisis still looms in the near future

emperatures have been flat for 15 years—nobody can properly explain it,” the Wall Street Journal says. “Global warming ‘pause’ may last for 20 more years, and Arctic sea ice has already started to recover,” the Daily Mail says. Such reassuring claims about climate abound in the popular media, but they are misleading at best. Global warming continues unabated, and it remains an urgent problem.

The misunderstanding stems from data showing that during the past decade there was a slowing in the rate at which the earth's average surface temperature had been increasing. The event is commonly referred to as “the pause,” but that is a misnomer: temperatures still rose, just not as fast as during the prior decade. The important question is, What does the short-term slowdown portend for how the world may warm in the future?

The Intergovernmental Panel on Climate Change (IPCC) is charged with answering such questions. In response to the data, the IPCC in its September 2013 report lowered one aspect of its prediction for future warming. Its forecasts, released every five to seven years, drive climate policy worldwide, so even the small change raised debate over how fast the planet is warming and how much time we have to stop it. The IPCC has not yet weighed in on the impacts of the warming or how to mitigate it, which it will do in reports that were due this March and April. Yet I have done some calculations that I think can answer those questions now: If the world keeps burning fossil fuels at the current rate, it will cross a threshold into environmental ruin by 2036. The “faux pause” could buy the planet a few extra years beyond that date to reduce greenhouse gas emissions and avoid the crossover—but only a few.

A Sensitive Debate

The dramatic nature of global warming captured world attention in 2001, when the IPCC published a graph that my co-authors and I devised, which became known as the “hockey stick.” The shaft of the stick, horizontal and sloping gently downward from left to right, indicated only modest changes in Northern Hemisphere temperature for almost 1,000 years—as far back as our data went. The upturned blade of the stick, at the right, indicated an abrupt and unprecedented rise since the mid-1800s. The graph became a lightning rod in the climate change debate, and I, as a result, reluctantly became a public figure. In its September 2013 report, the IPCC extended the stick back in time, concluding that the recent warming was likely unprecedented for at least 1,400 years.

Although the earth has experienced exceptional warming over the past century, to estimate how much more will occur we need to know how temperature will respond to the ongoing human-caused rise in atmospheric greenhouse gases, primarily carbon dioxide. Scientists call this responsiveness “equilibrium climate sensitivity” (ECS). ECS is a common measure of the heating effect of greenhouse gases. It represents the warming at the earth's surface that is expected after the concentration of CO2 in the atmosphere doubles and the climate subsequently stabilizes (reaches equilibrium).

The preindustrial level of CO2 was about 280 parts per million (ppm), so double is roughly 560 ppm. Scientists expect this doubling to occur later this century if nations continue to burn fossil fuels as they do now—the “business as usual” scenario—instead of curtailing fossil-fuel use. The more sensitive the atmosphere is to a rise in CO2, the higher the ECS, and the faster the temperature will rise. ECS is shorthand for the amount of warming expected, given a particular fossil-fuel emissions scenario.

It is difficult to determine an exact value of ECS because warming is affected by feedback mechanisms, including clouds, ice and other factors. Different modeling groups come to different conclusions on what the precise effects of these feedbacks may be. Clouds could be the most significant. They can have both a cooling effect, by blocking out incoming sunlight, and a warming effect, by absorbing some of the heat energy that the earth sends out toward space. Which of these effects dominates depends on the type, distribution and altitude of the clouds—difficult for climate models to predict. Other feedback factors relate to how much water vapor there will be in a warmer atmosphere and how fast sea ice and continental ice sheets will melt.

Because the nature of these feedback factors is uncertain, the IPCC provides a range for ECS, rather than a single number. In the September report—the IPCC's fifth major assessment—the panel settled on a range of 1.5 to 4.5 degrees Celsius (roughly three to eight degrees Fahrenheit). The IPCC had lowered the bottom end of the range, down from the two degrees C it had set in its Fourth Assessment Report, issued in 2007. The IPCC based the lowered bound on one narrow line of evidence: the slowing of surface warming during the past decade—yes, the faux pause.

Many climate scientists—myself included—think that a single decade is too brief to accurately measure global warming and that the IPCC was unduly influenced by this one, short-term number. Furthermore, other explanations for the speed bump do not contradict the preponderance of evidence that suggests that temperatures will continue to rise. For example, the accumulated effect of volcanic eruptions during the past decade, including the Icelandic volcano with the impossible name, Eyjafjallajökull, may have had a greater cooling effect on the earth's surface than has been accounted for in most climate model simulations. There was also a slight but measurable decrease in the sun's output that was not taken into account in the IPCC's simulations.

Natural variability in the amount of heat the oceans absorb may have played a role. In the latter half of the decade, La Niña conditions persisted in the eastern and central tropical Pacific, keeping global surface temperatures about 0.1 degree C colder than average—a small effect compared with long-term global warming but a substantial one over a decade. Finally, one recent study suggests that incomplete sampling of Arctic temperatures led to underestimation of how much the globe actually warmed.

None of these plausible explanations would imply that climate is less sensitive to greenhouse gases. Other measurements also do not support the IPCC's revised lower bound of 1.5 degrees C. When all the forms of evidence are combined, they point to a most likely value for ECS that is close to three degrees C. And as it turns out, the climate models the IPCC actually used in its Fifth Assessment Report imply an even higher value of 3.2 degrees C. The IPCC's lower bound for ECS, in other words, probably does not have much significance for future world climate—and neither does the faux pause.

For argument's sake, however, let us take the pause at face value. What would it mean if the actual ECS were half a degree lower than previously thought? Would it change the risks presented by business-as-usual fossil-fuel burning? How quickly would the earth cross the critical threshold?

A Date with Destiny: 2036

Most scientists concur that two degrees C of warming above the temperature during preindustrial time would harm all sectors of civilization—food, water, health, land, national security, energy and economic prosperity. ECS is a guide to when that will happen if we continue emitting CO2 at our business-as-usual pace.

I recently calculated hypothetical future temperatures by plugging different ECS values into a so-called energy balance model, which scientists use to investigate possible climate scenarios. The computer model determines how the average surface temperature responds to changing natural factors, such as volcanoes and the sun, and human factors—greenhouse gases, aerosol pollutants, and so on. (Although climate models have critics, they reflect our best ability to describe how the climate system works, based on physics, chemistry and biology. And they have a proved track record: for example, the actual warming in recent years was accurately predicted by the models decades ago.)

I then instructed the model to project forward under the assumption of business-as-usual greenhouse gas emissions. I ran the model again and again, for ECS values ranging from the IPCC's lower bound (1.5 degrees C) to its upper bound (4.5 degrees C). The curves for an ECS of 2.5 degrees and three degrees C fit the instrument readings most closely. The curves for a substantially lower (1.5 degrees C) and higher (4.5 degrees C) ECS did not fit the recent instrumental record at all, reinforcing the notion that they are not realistic.

To my wonder, I found that for an ECS of three degrees C, our planet would cross the dangerous warming threshold of two degrees C in 2036, only 22 years from now. When I considered the lower ECS value of 2.5 degrees C, the world would cross the threshold in 2046, just 10 years later [see graph on pages 78 and 79].

So even if we accept a lower ECS value, it hardly signals the end of global warming or even a pause. Instead it simply buys us a little bit of time—potentially valuable time—to prevent our planet from crossing the threshold.

Cautious Optimism

These findings have implications for what we all must do to prevent disaster. An ECS of three degrees C means that if we are to limit global warming to below two degrees C forever, we need to keep CO2 concentrations far below twice preindustrial levels, closer to 450 ppm. Ironically, if the world burns significantly less coal, that would lessen CO2 emissions but also reduce aerosols in the atmosphere that block the sun (such as sulfate particulates), so we would have to limit CO2 to below roughly 405 ppm.

We are well on our way to surpassing these limits. In 2013 atmospheric CO2 briefly reached 400 ppm for the first time in recorded history—and perhaps for the first time in millions of years, according to geologic evidence. To avoid breaching the 405-ppm threshold, fossil-fuel burning would essentially have to cease immediately. To avoid the 450-ppm threshold, global carbon emissions could rise only for a few more years and then would have to ramp down by several percent a year. That is a tall task. If the ECS is indeed 2.5 degrees C, it will make that goal a bit easier.

Even so, there is considerable reason for concern. The conclusion that limiting CO2 below 450 ppm will prevent warming beyond two degrees C is based on a conservative definition of climate sensitivity that considers only the so-called fast feedbacks in the climate system, such as changes in clouds, water vapor and melting sea ice. Some climate scientists, including James E. Hansen, former head of the nasa Goddard Institute for Space Studies, say we must also consider slower feedbacks such as changes in the continental ice sheets. When these are taken into account, Hansen and others maintain, we need to get back down to the lower level of CO2 that existed during the mid-20th century—about 350 ppm. That would require widespread deployment of expensive “air capture” technology that actively removes CO2 from the atmosphere.

Furthermore, the notion that two degrees C of warming is a “safe” limit is subjective. It is based on when most of the globe will be exposed to potentially irreversible climate changes. Yet destructive change has already arrived in some regions. In the Arctic, loss of sea ice and thawing permafrost are wreaking havoc on indigenous peoples and ecosystems. In low-lying island nations, land and freshwater are disappearing because of rising sea levels and erosion. For these regions, current warming, and the further warming (at least 0.5 degree C) guaranteed by CO2 already emitted, constitutes damaging climate change today.

Let us hope that a lower climate sensitivity of 2.5 degrees C turns out to be correct. If so, it offers cautious optimism. It provides encouragement that we can avert irreparable harm to our planet. That is, if—and only if—we accept the urgency of making a transition away from our reliance on fossil fuels for energy. your social media marketing partner


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+13 # universlman 2014-03-20 18:41
With public opinion split in the US as to whether warming is even happening, and the loudest angry voices in the media calling for no action, it is not likely that we will start on remedial action before it is far too late. This is why all of the windmills on the hilltops are made in Norway. We are buying most of our solar panels from Germany. Americans pioneered these inventions in the 70s, but their voices were drowned out back then by the well funded fossil fuel lobby, and it is happening again today (log on to learn more.) Follow the money folks, not the people working on renewables. Those people will be relentlessly marginalized, targeted, and Solyndralized.
+1 # RobertMStahl 2014-03-21 15:54
One might consider that western civilization's historic fixation with science might be more than just a gambling scenario in the long run, or be more than just an old guard playing with the toys of modern chemistry (i.e. where explosives define all changing of the guard).

Compassion might just be something smart instead of this pile of 'blue' chips on the baccarat table of life, and something untried.

Undeniably, there is the paradigm shifting work of the late Lynn Margulis, along with the tectonic changing thought process, or, advancement of James Lovelock indicating the significance the disequilibrium of this planet that is thermodynamical ly unstable against the backdrop of an, otherwise, hostile environment (the universe). This maintenance is a result of the overall chemistry of life on earth, having done so against a sun having heated up a lot along the way, consistently.

The thing is, not ignoring what Lovelock said about PETM, there is the flawless and massive work, and on a daily basis for years upon years, of the author of the Grand Unified Theory of Classical Physics (GUTCP) with progress being made presently in the field of 'fireless' pollution-free energy, and cheap. It is reliant on the most stable element in the universe, perhaps the most abundant, hydrogen in the state of dark matter known to all. Meanwhile, the shock of learning remains, the trauma that prevents us from seeing anything new. Really, most of what is old hasn't been doing much good.
+10 # Walter J Smith 2014-03-20 21:18
RE: The IPCC's "...forecasts, released every five to seven years, drive climate policy worldwide...."

I don't know what planet you are from, but those reports have absolutely nil impact on Wall Street's US Government.

The US Government "policy" on climate challenges is a pile of empty rhetorical soup.
+12 # RnR 2014-03-20 23:22
Our federal government will do nothing other than enable Monsanto's spreading of its poison world wide.

They are too stupid and too infiltrated by neocons.
+13 # alnbarthel 2014-03-21 03:17
As long as our Federal and state elected officials are owned by the corporate world, their only motive for governing will be the short term economic benefits of those very corporations. The junk science of the Koch brothers and their likes will also be allowed to informed their decisions. I for one do not hold out much hope for the future of humanity.
-7 # slocan 2014-03-21 08:37
Blah, blah blah. How many of these take off reports have we seen that spin the same information just like CNN. By having a date like 2036 where somehow everything will go Bang it sets up a potential disconnect in the . Ecosystem degradation will be much farther along and that is a much greater worry that severe storms or droughts which will occur relatively haphazardly around the world. Climate change is more like the icing on the death of the present form of Nature that exists on Earth. The article's author plays with the model and here I find the greatest disconnect. I do not find his feelings, his heart feelings about this much at all. He remains , pseudo scientific and ends with well maybe if the rise isn't too bad we will be able to avert disaster. No discussion of what stories we have such as economic growth forever, technology as a concrete weight sinking us, and a ongoing world view that says Nature is here for our consumption. His ending is completely anthropocentric .
-11 # Malcolm 2014-03-21 10:11
Mike, you said, "Although climate models have critics, they reflect our best ability to describe how the climate system works, based on physics, chemistry and biology. And they have a proved track record: for example, the actual warming in recent years was accurately predicted by the models decades ago.)"

This is absolutely unsupportable. The very "hockey stick" you cite disproves that the models "accurately predicted" the leveling off of temperature rise.

The models failed; you can make all the excuses you want about the failure (volcanoes, La Niña, solar input, ocean heat absorption, etc.) but the fact is that they DID NOT predict this hiatus. It's hard to imagine you're even making that claim! (Or maybe by "recent years", you meant recent years EXCEPT FOR THE LAST 15?)
-4 # Malcolm 2014-03-21 16:17
Wowie zombie-six thumbs down! Without a single response. Did I make an error of some sort, or do y'all simply dislike hearing the truth?
-4 # Malcolm 2014-03-22 02:10
Obviously, it's the latter. Would it help to apologize for bursting y'all's bubble?

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