The Earth’s climate system may be far more sensitive to increasing amounts of greenhouse gases than many recent studies have found, which implies that steeper and faster emissions cuts will be necessary to keep warming below dangerous levels. The new research, which appeared this week in the journal Nature Climate Change, questions a central assumption of most of the studies investigating a scientific concept generally referred to as “climate sensitivity.”
The research found that the Earth is likely to experience nearly 20% more warming over the next several decades than past estimates have projected. Those previous studies were based in large part on recent surface temperature trends.
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Climate sensitivity and a related measure known as the “transient climate response" are both metrics that capture how much the planet will warm in response to increases in manmade greenhouse gas emissions and other pollutants.
“It is just a kind of yardstick for how much climate change you’d get if you pushed the system a certain amount,” said the study’s author, Drew Shindell of NASA’s Goddard Institute for Space Studies, in an interview.
Technically speaking, the transient climate response calculation determines how much global temperatures will change as atmospheric carbon dioxide levels continue to increase — at about 1% per year — until the total amount of atmospheric carbon dioxide has doubled.
Thanks to human activities such as burning fossil fuels for energy, the amount of planet warming greenhouse gases in the atmosphere have skyrocketed to unprecedented levels in human history.
For example, in 2013, the amount of carbon dioxide, the primary long-lived greenhouse gas, in the atmosphere hit 400 parts per million, which is likely the highest level in at least 800,000 years.
Yet despite this increase in greenhouse gases, the pace of global warming has slowed during the past two decades. This has led some researchers to conclude that perhaps the climate isn’t as sensitive to greenhouse gases as previously assumed. For example, in a major report released last year, the United Nations Intergovernmental Panel on Climate Change (IPCC) widened its estimate of climate sensitivity to include lower estimates than in its previous report in 2007, and numerous media reports indicated that scientists had lowered their projections of future warming.
While the most recent IPCC report found that the likely range of climate sensitivity is between 1.8 degrees and 4.5 degrees Fahrenheit, the new study found that the most likely figure is higher, at a best estimate of 3.06 degrees Fahrenheit, with a range of between 2.34 degrees and 5.76 degrees Fahrenheit. Importantly, the study also found the sensitivity is unlikely to be below 2.34 degrees Fahrenheit, effectively rejecting the low-end IPCC estimate.
Six decades of surface temperature departures from average. Credit: NASA.
This is only a best estimate for how the climate would warm in response to a doubling of carbon dioxide. In reality, most projections show we are on course to more than double such atmospheric concentrations by 2100.
For the new study, NASA's Shindell used more sophisticated computer models and found that a key assumption in most climate sensitivity studies causes these estimates to be too low.
Climate scientists have long used relatively simple computer models to estimate how much the climate would warm due to greenhouse gas emissions and emissions of other substances, such as small particle pollution from power plants and vehicles, known as aerosols. These models assume that the temperature change in response to such emissions is proportional to how much those emissions change the heating of the planet, no matter where the emissions take place.
However, Shindell’s research found that aerosols, which are distributed unevenly around the world, have a larger impact on the climate than previously assumed; this is because most aerosols are emitted in the northern hemisphere, where there is more land area as well as snow and ice coverage. Aerosols can come from natural sources, such as sea spray, as well as manmade sources, such as power plants, wildfires and vehicles. Some aerosols cause a net warming of the planet, whereas others cause cooling that offsets some of the influence of carbon dioxide and other planet-warming gases.
Much of the pollution that has led to the air pollution crisis in parts of China is in the form of aerosols, for example.
“The assumption that whatever forcing you put in gives you the same answer doesn’t hold,” Shindell said in an interview.
In general, land, snow and ice adjust to atmospheric changes far more quickly than the oceans of the world, which explains the larger climate response in the northern hemisphere.
Shindell said it is now clear that more warming is likely to take place in the next several decades, compared to findings from other recent studies.
“All the evidence aligns to point towards the higher end,” he said. This is an important finding, since it implies that sharp and immediate emissions cuts are needed in order to avoid such warming, whereas a lower estimate of climate sensitivity would imply that more gradual cuts might be sufficient to keep warming below dangerous levels set by global policymakers.
“It’s nice to have some certainty,” he said. “Even though I would’ve preferred the climate sensitivity to be low, it’s better that we have a sense that we really understand what is going on much more clearly.”
Reto Knutti, a professor for the Institute for Atmospheric and Climate Science in Zurich, who was not involved in Shindell’s study, said the research is significant and “closes a big gap in our understanding.”
“In my view, the results are solid,” he said in an email conversation. “The good news would be that we have reconciled the differences between various methods, the bad news is that we are unlikely to be lucky and experience low warming.”
Myles Allen, a climate researcher at Oxford University, also told Mashable that Shindell’s results match other recent estimates using different scientific approaches, and don’t support lower-end projections of warming. “There is lots of evidence accumulating that the fuss over the models being biased high was probably over-done,” Allen said in an email.
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