Only recently has it become possible to detect the effects of human-induced greenhouse gas emissions on global temperatures. However, by examining data on a regional basis, climate scientists have found indications of global warming dating to the 1940s.
Greenhouse gases have been increasing in the atmosphere since the Industrial Revolution, but initially their effects were too small to be distinguished from random fluctuations. Now, a paper in Environmental Research Letters explores patterns in average and extreme temperatures region by region, as well as looks at the intensity of rainfall events. The authors sought the point where a particular measure first moved outside expected values as a result of human-induced global warming.
Modeling of when global warming signatures should become, or have become, detectable in different regions from 23 model simulations. (a) and (b) mean surface air temperature, (c) and (d) highest daily maximum temperature, (e) and (f) lowest daily minimum temperature, (g) and (h) total precipitation, and (i) and (j) maximum precipitation in a day. First column: June-August; second: December-February. Credit: King et al.
First author Dr. Andrew King of the University of New South Wales told IFLScience, “We did a statistical test of distributions. We had a rigorous requirement that values had to remain well outside natural distributions for all subsequent periods. They couldn't just pop out and pop back in."
"Remarkably our research shows that you could already see clear signs of global warming in the tropics by the 1960s but in parts of Australia, South East Asia and Africa it was visible as early as the 1940s," King said in a statement.
The wavelengths that carbon dioxide captures should cause more rapid warming in colder environments, and the fastest warming observed so far has been in the Antarctic Peninsula and Alaska. Despite this, King found that the first warming that could be confidently attributed to human influences was in the tropics, while the same indications could not be established from polar data until 40 years later.
“Inter-annual variability is very low in the tropics,” he told IFLScience, so warming changes stand out more. King added that for this reason the effects of warming can also be more severe in the tropics. "Polar flora and fauna are adapted to wider temperature changes,” but the same is not true for the tropics, where small variations can have a bigger impact.
In any given region, average seasonal temperatures showed a clear pattern before extremes. King told IFLScience this was not surprising: “For average temperatures we had 90 values in a season, whereas the extremes were just one per season, so there was much more noise to hide the trend.”
Changes in rainfall lagged even further behind, and indeed are generally hard to discern definitively even today. “We expect the first heavy precipitation events with a clear global warming signal will appear during winters in Russia, Canada and northern Europe over the next 10-30 years,” said co-author Dr. Ed Hawkins of the University of Reading.
Today, temperatures show a clear anthropogenic influence on almost every part of the Earth. The notable exception is the eastern United States, where political climate change denial is matched, and perhaps enhanced, by an absence of a clear trend, although King anticipates one will emerge in the next decade.