Climate models have accurately predicted global heating for the past 50 years, a study has found.
The findings confirm that since as early as 1970, climate scientists have had a solid fundamental understanding of the Earth’s climate system and the ability to project how it will respond to continued increases in the greenhouse effect. Since climate models have accurately anticipated global temperature changes so far, we can expect projections of future warming to be reliable as well.
The research examines the accuracy of 17 models published over the past five decades, beginning with a 1970 study and including 1981 and 1988 models led by James Hansen, the former Nasa climatologist who testified to the US Senate in 1988 about the impacts of anthropogenic global heating. The study also includes the first four reports by the UN’s intergovernmental panel on climate change (IPCC).
“We found that climate models – even those published back in the 1970s – did remarkably well, with 14 out of the 17 model projections indistinguishable from what actually occurred,” said Zeke Hausfather, of the University of California, Berkeley, and lead author of the paper.
Based on modern climate model projections, if countries follow through with current and pledged climate policies, the world is on track for about 3C of warming above pre-industrial temperatures by 2100 – a situation the IPCC and others predict would be catastrophic.
The challenge in evaluating climate model accuracy lies in the fact that due to computing power limitations, simulations are only run for a few specific future greenhouse gas emissions scenarios. There are an infinite number of such possible scenarios, but real-world emissions will follow only one path, and it will never exactly match the few scenarios input into climate models. Thus, if Earth warms less than in a climate model projection, it does not necessarily mean the model was inaccurate.
Put simply, climate scientists are not in the business of predicting human fossil fuel consumption but are attempting to accurately simulate how the climate will change in response to a given rise in greenhouse gas emissions.
“Future emissions depend on human behaviour, not physical systems, and climate models should be evaluated on their physics rather than the future emission projections,” said Hausfather.
In nearly half of the model projections examined in the paper, the input scenarios were significantly different from the real-world changes in greenhouse gas emissions. As a result, projected temperature changes were only consistent with observed global warming in 10 of the 17 models, with four projecting more warming and three projecting less than subsequently occurred.
However, the study authors addressed these inconsistencies by evaluating the change in temperature per change in “radiative forcing” – the global energy imbalance caused by the increased greenhouse effect and other factors – in models against what happened in the real world. This metric reveals whether the climate models are accurately producing the temperature response to a given emissions change – in essence, whether are accurately simulating the physical response of Earth’s climate system. With this factored in, 14 of the 17 models were accurate.
“The rate of warming we are experiencing today is pretty much exactly what past climate models projected it would be,” said Hausfather.
Those who oppose policies to limit the impacts of global heating have long sought to undermine the credibility of climate models. If the model projections are considered unreliable, they argue, then we do not know how urgent slowing global warming is. As a result, “climate models are unreliable” has become a popular myth propagated by climate deniers.
The latest study adds to the body of evidence supporting the accuracy of climate models, and will be welcomed by those arguing that more aggressive climate policies are needed to avoid dangerous levels of global warming. The UN climate summit in Glasgow in 2020 will be crucial, as countries will be expected to commit to scaling up the emission reductions that were pledged in the 2015 Paris agreement on climate change.