Posted on 27 July 2016
The discrepancies are due to data inaccuracies that falsely suggest historical climate records and current climate models do not match.
The study, led by NASA in collaboration with the Universities of York and Reading, explains why projections of future climate, based solely on historical records, estimate lower rates of warming than predictions from climate models today.
After many years of acknowledging this discrepancy, scientists have now demonstrated that measured warming since the 19th century actually agrees with climate models despite earlier reports that the data favoured less warming in response to greenhouse gas emissions.
Calculations using measured climate change since the 19th century previously suggested that climate models show around a third too much warming in response to greenhouse gas increases. This, however, conflicted with satellite measurements of climate processes and evidence from the deep past, which pointed toward more warming.
Dr Kevin Cowtan, from the University of York’s Department of Chemistry, said: “The problem was due to a mismatch between the way temperatures are calculated in models versus the real world. We can’t go back and collect more data from the past, but we can measure the climate models in the same places where we had real weather stations.”
The Arctic is warming faster than the rest of Earth, but there are fewer historic temperature readings from there than from lower latitudes because it is so inaccessible. A data set with fewer Arctic temperature measurements naturally shows less warming than a climate model that fully represents the Arctic.
The new study also accounted for two other issues. First, the historical data mix air and water temperatures, whereas model results often report air temperatures only. This quirk also skews the historical record toward the cool side, because water warms less than air.
The final issue is that there was considerably more Arctic sea ice when temperature records began in the 1860s. As sea ice declined, ships started measuring water temperatures where previously only air temperatures from coastal weather stations had been available.
Dr Cowtan added: “We have known about this problem for a while, but this is the first time we have been able to explain the puzzle. When you apply a like-with-like calculation to models and climate records, there is no significant disagreement between the models and observations.
“This is important because it means that data used to inform vital environmental commitments, such as the Paris climate agreement, which aims to limit global warming to below 2 °C, is in fact supported by both the climate models and historical observations.”
The research is published in Nature Climate Change.