Climate sensitivity adjustment (N&V)

The previous low-end calculations of climate sensitivity (temperature response to the changes in atmospheric composition) of less than 1.3 degrees Celsius are likely incorrect, reports a paper published online in Nature Climate Change. The reason for this low estimate is that calculations of climate sensitivity did not account for the greater net cooling effect of atmospheric particles and ozone as opposed to the warming effect of carbon dioxide.

Understanding how climate sensitivity is affected by changes in atmospheric composition is important for accurate projections of temperature changes resulting from anthropogenic emissions. However, despite increasing greenhouse gases, surface temperatures have increased relatively slowly over the past 10-15 years, leading to estimates of climate sensitivity based on those observations and simple climate models that were at the low-end of projections from complex climate models.

Drew Shindell analysed results from the latest climate model intercomparison study (CMIP5) to understand how climate sensitivity varies depending on atmospheric composition. Previous studies using simple models have applied the same sensitivity weighting to atmospheric particles and ozone as applied to carbon dioxide, and a recent study reported a transient climate response of 1.3 degrees Celsius. However, by taking into account the varying effects of particles, ozone and carbon dioxide, Shindell reports a transient climate response of 1.7 degrees Celsius. This is in line with more complex model results.

In an accompanying News and Views piece, David Stainforth writes, “this new estimate for transient climate response comes about by questioning a widely-made implicit assumption in the application of simple climate models” and notes that it is important for simple models to accurately project temperature changes as they are the models commonly used for policy-relevant research.