Despite Debate, Estimates of Humanity’s Impact on Climate Change Are Accurate
A new study reconciles observed changes to the climate and computer model predictions, showing that they are more in agreement than they seem.
Climate change contrarians have used many arguments to cast doubt on the scientific consensus that the atmosphere is warming and humans are to blame.
They’ve alleged that tens of thousands of scientists, working across dozens of different disciplines, have organized a vast conspiracy to manipulate data. They’ve said more carbon dioxide in the atmosphere will actually be a benefit because it will boost agricultural production. And they’ve even said that global warming has stopped in recent years.
But their primary tactic these days appears to be to acknowledge that the concentration of greenhouse gases in the atmosphere is increasing due to human causes, but suggest that the scientific community just can’t pin down what impact those emissions will have on the climate system.
Trump’s EPA administrator Scott Pruitt made that claim during his Senate confirmation hearing, as did former ExxonMobil CEO turned Secretary of State Rex Tillerson.
Kyle Armour of the University of Washington, writing in Nature Climate Change, says that, actually, the scientific community has done quite well at developing accurate predictions of how sensitive the atmosphere and oceans are to carbon pollution.
The debate, he says, pivots around a number called equilibrium climate sensitivity (ECS), which is the temperature change that scientists expect to occur with a doubling of carbon dioxide in the atmosphere compared to before the Industrial Revolution.
“This is the number that’s been essentially at the heart of climate change predictions for decades now,” he said. “It has a lot of policy relevance. If this number is high, we have a very sensitive Earth that will warm up a lot in response to greenhouse gases. If the number is low, we have a less sensitive climate system that would warm up a lot less.”
Put another way, climate sensitivity tells us how much we need to limit greenhouse gas emissions in order to stay well below an average global temperature increase of 2°C compared to the mid-19th century, which many scientists say is a dangerous threshold to pass.
The problem is observations collected over the past 100 years have shown the climate to be less sensitive to increasing greenhouse gas concentrations than computer models of the climate have predicted.
And that’s where Armour got to work.
“It appeared that newer observations suggested a fairly low climate sensitivity in the range of about 2°C for a doubling of CO2, whereas the models suggested a higher climate sensitivity more in the range of 3°C for a doubling of CO2,” he said. “What I was interested in with the study was basically how much of the discrepancy between the observations and the models could be explained by the fact that climate sensitivity changes over time.”
There’s an incredible amount of inertia built into Earth’s climate system. The increased amount of energy trapped in the atmosphere due to all of the greenhouse gas emissions from our coal-fired power plants and internal combustion engines takes many decades to fully take effect.
And the climate system works in complex ways. When rising air temperatures in the Arctic melt summer sea ice, for example, it creates larger areas of dark, open ocean which absorb rather than reflect sunlight, which in turn causes more global warming. That type of feedback loop combined with the inertia of the climate system means climate change doesn’t move along smooth, linear plots on a line graph.
Armour found that while observations might show climate sensitivity to be on the lower end, the models are picking up quite well the amount of warming that emerges over the long-term.
“The conclusions are really two-fold,” said Armour. “When the models are treated consistently with the observations, that is measuring climate sensitivity within the models in the same way you would with the observations, it brings that value of sensitivity in the models downward, meaning they’re not too sensitive.”
“But the other way you could view the results,” he added, “is that the model range of future warming is rather realistic, meaning that our apparent climate sensitivity we get from the observations can be expected to increase in the future, meaning more global warming than you might naïvely expect from taking just that low value that people have been talking about from the observations.”
Gavin Schmidt, a NASA climate scientist not involved in the study, said Armour's findings complemented the results of a 2016 report on climate sensitivity conducted by researchers at NASA and Columbia University.
“What Armour finds is that [feedback loops] do change and in such a way that if you estimate the final sensitivity from just the early period you will end up underestimating the ECS,” he said in an email. “This is important because that’s almost exactly what’s happening when we try and use recent temperature trends to estimate the ECS. Those estimates have tended to come in lower than others, in ways that aren’t consistent with our understandings of processes or paleoclimate. So this result makes those studies much more consistent with other methods.”
Armour pointed to a pair of feedbacks that have yet to take effect, but are likely to lead to significant levels of warming.
“Over the next several decades to centuries we expect the Southern Ocean to warm up almost as much as the Arctic and get that big, positive feedback to start kicking in,” he said. “So it’s really that delay in the positive feedbacks kicking in that causes this increase in climate sensitivity into the future.”
That feedback in the southern hemisphere is projected to occur when ocean temperatures in the Southern Ocean warm, melting sea ice, like is already happening in the Arctic, causing the ocean to absorb more sunlight and leading to greater amounts warming.
Increased warming in the eastern tropical Pacific over the next several decades, he added, is is likely to diminish cloud cover in the region. That cloud cover, like sea ice in the Arctic and Southern Ocean, reflects incoming sunlight.
“Initially in the very early stages of global warming, like today, like we’ve seen over the last hundred years, your clouds in the eastern tropical Pacific are actually acting to limit global warming, it has a negative, damping impact,” he said. “But in the future, say over the next several decades, we expect those to be a positive feedback, enhancing global warming.”
What Armour didn’t find was any suggestion that Earth’s climate might somehow absorb all of our increased greenhouse gas emissions and warm less than any climate model has predicted.
“There are no examples of models showing a decrease in sensitivity by any significant amount,” he said. “That range of predicted warming in the future, is actually, as far as we can tell, pretty accurate.”
And those predictions, whether gleaned from UN reports or NASA or any other peer-reviewed account, warn of dangerous changes to the climate, which could imperil millions of people and future generations.
“This is a good result for the scientists — because it closes a hole of inconsistency — but in terms of the ‘public’ argument, this won’t matter,” said NASA’s Schmidt. “The contrarians have been ignoring studies like this for years, and I doubt they will stop doing so now.”
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