Sense and climate sensitivity – more evidence we're in for a hot future

Arguments that the climate is relatively insensitive to the increased greenhouse effect have become the last best chance for climate contrarians, but a new study from Texas A&M University hammers a big nail in the coffin of that argument.

With overwhelming evidence that humans are the main cause of global warming, and with arguments that modest warming is beneficial falling apart, those who oppose climate solutions are forced to put their eggs in the 'low climate sensitivity' basket. If the Earth's climate is less sensitive to the increased greenhouse effect than most of the available evidence indicates, then perhaps there's not quite as much urgency to tackle the threat of global warming, they argue.

The IPCC shifts on climate sensitivity

The latest IPCC report gave contrarians a glimmer of hope in this area. The fourth IPCC report in 2007 estimated that the planet will warm between 2 and 4.5°C warming in response to a doubling of the amount of CO2 in the atmosphere, with a best estimate of 3°C. At the time, all of the different approaches to estimate climate sensitivity (using historical data, climate models, and recent instrumental data) were in good agreement.

However, in recent years, a few studies using a combination of instrumental data and simple climate models have arrived at lower estimates. This caused the IPCC in its fifth report in 2014 to reduce the lower end of its estimated climate sensitivity range, back to 1.5 to 4.5°C in response to doubled CO2 (the same range as its reports estimated prior to 2007). Due to the newfound disagreement between the different methods, the IPCC also dropped its best estimate altogether.

The 'instrumental' climate sensitivity estimates were something of a puzzle. Why were they no longer in agreement with estimates from historical climate changes and climate models, which still consistently arrived at climate sensitivity estimates between 2 and 4.5°C? John Abraham reported on a study by Drew Shindell at NASA that may have hit on the answer.

Shindell suggests the source of the discrepancy

These 'instrumental' estimates were assuming that the Earth's climate is equally sensitive to all external temperature influences. However, while greenhouse gases are well mixed throughout the atmosphere, aerosols and ozone are concentrated more in the northern hemisphere. Here the temperature response to energy imbalances is more sensitive, as world-renowned climate scientist James Hansen noted in a 1997 paper,

“A forcing at high latitudes yields a larger response than a forcing at low latitudes. This is expected because of the sea ice feedback at high latitudes and the more stable lapse rate at high latitudes

Shindell found that correcting for this faulty assumption brought the 'instrumental' climate sensitivity estimates much higher in the models he looked at.

Kummer & Dessler build on Shindell's results

In their study, Kummer & Dessler took Shindell's approach one step further. Dessler told me,

"I view my paper as a follow-on to Shindell's paper. What he showed in his paper was that climate models respond more strongly to forcing from aerosols and ozone. What we show in our paper is that if we take his result, and re-analyze the 20th-century observational record then we get a higher climate sensitivity than [studies] which assumed that all forcing was equally effective. Taking efficacy into account, our climate sensitivity is right in the middle of the values derived from other sources. So this allows us to bridge the gap between the various estimates of climate sensitivity and converge on a value around 3°C."

Dessler also explains the study's approach and results in the following 4-minute video.

In short, Shindell showed that according to models, the climate is significantly more sensitive to changes in aerosols and ozone than greenhouse gases, perhaps by as much as 50%. Kummer & Dessler showed that if the climate is 33% more sensitive to changes in aerosols and ozone, then the 'instrumental' estimates are right in line with those derived from historical climate changes and global climate models, with a best estimate of 3°C warming in response to a doubling of atmospheric CO2.

What does this mean for future global warming?

If we continue on our current business-as-usual path, we're on track for close to two doublings of atmospheric CO2. If climate sensitivity is 3°C for doubled CO2, that gives us a best estimate of about 5°C warming above pre-industrial temperatures by 2100, while even 2°C is considered dangerous.

Posted by on Monday, 12 May, 2014

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