Climate scientists have a surprising habit: They often underplay the climate threat. In 2007 a team led by Stefan Rahmstorf compared actual observations with projections made by theoretical models for three key climate variables: atmospheric carbon dioxide, global average temperature and sea-level rise. While the projections got CO2 levels right, they were low for real temperature and sea-level rise. In 2008 Roger Pielke, Jr., found that sea-level rise was greater than forecast in two of three prior Intergovernmental Panel on Climate Change reports. In 2009 a review of hundreds of papers on climate change identified several areas where scientists had lowballed event predictions but none in which they had overestimated them.
In 2013 researcher Keynyn Brysse, then at the University of California, San Diego, along with other colleagues and me, pointed out that these underestimates represent a kind of bias. Scientists preferred to use lower projections in order not to be accused of making exaggerated or dramatic claims. It is possible to believe that scientists would have taken corrective actions by now if they had cited the articles reporting the underestimates.
But recent studies of Arctic warming suggest that the problem may not have gone away. Scientists have known for a long time that Arctic ice reflects sunlight and heats the planet. Global warming has caused polar ice to melt, which causes the Arctic Ocean to absorb more heat, which causes it to warm more, which melts even more ice. It shouldn’t surprise anyone that the region is warming quickly. Scientists have been taken by surprise at how quickly the region is heating up.
A recent study led by Mika Rantanen of the Finnish Meteorological Institute found that, since 1979, the Arctic has warmed nearly four times faster than Earth as a whole. This is a remarkable effect that few climate models have predicted.
Model results are typically reported as the averages of many runs of a set of similar models, referred to as ensembles. These Arctic temperature observations are not only warmer that all major ensemble averages, but also outside the ensemble envelope in some cases. In one very large and highly respected one–the Max Planck Institute Grand Ensemble–the observed warming for 1979-2021 is entirely beyond the results. Some observations in the real world are more hot than projections.
This has several implications. It reminds us that averages are not always accurate. Extreme outcomes are possible, but they do happen and can be crucial for assessing risk. It also suggests that climate models might continue to underestimate key climate effects.
Admittedly, the observations might be wrong; measuring the temperature of the region is notoriously difficult, in part because of sparse sensor coverage over the Arctic Ocean. Scientists may have used different time periods to analyze the data or used conflicting definitions for Arctic boundaries. It could also be that their subconscious bias towards playing things down was playing up.
We’ve heard a lot in recent years about subconscious bias in relation to race and gender discrimination. However, subconscious bias can also be caused by defensiveness. Even now scientists continue to be accused of exaggerating climate risks by prominent figures who get outsized media attention. Scientists who have consciously considered this concern may subconsciously bias their models to be unrealistically conservative.
If scientists have underestimated Arctic warming, they have likely minimized amounts of permafrost melting and methane release as well. And that could be truly dire because the permafrost holds about 1.5 billion metric tons of organic carbon, twice as much as now in the atmosphere. If that carbon were to be released quickly, it could lead to a runaway greenhouse effect. Scientists need to seriously examine whether their models continue underplaying critical aspects of climate change. Low estimates can give the false impression that we have more time than we actually have to fix the problem.