Recent past a poor guide to climate future

The Earth during the Last Glacial Maximum

Don’t look to the recent past to understand near-term global warming, “the spatial pattern of global warming in the most recent 40 years doesn't look like the long-term pattern we expect in the future. The recent past is a bad analogue for future global warming,” according to Kyle Armour, of the University of Washington.

Instead it is the more distant past that might hold important clues about the warming we may expect by the end of this century. Short-term climate cycles like those we have recently experienced, coupled with the effects of atmospheric pollution, suggest that we can’t use recent data to reliably predict the rest of this century.

The caution is expressed in a new study published in Science Advances. It  doesn’t alter the best-case warming scenario from doubling CO2 — about 2°C – but it does reduce the worst-case scenario from 5°C to 4°C.

This climate sensitivity estimate comes from looking at the recent paleoclimate record – a few tens of thousands of years ago – that includes long periods that were much warmer or colder than our current climate. This was caused by many factors, such as the influence of large ice sheets and greenhouse gases. Scientists take an interest in these past times because knowing, however roughly, what the past temperature changes were and what caused them, sheds light on what to expect in the future.

The researchers looked at the Last Glacial Maximum, a period 21,000 years ago when the Earth was on average 6°C cooler than today. Ice core records from the time show that atmospheric CO2 then was less than half of today’s levels, about 190 parts per million. Using new statistical techniques, the researchers have incorporated the paleoclimate data into climate models, producing temperature maps of the time.

The study shows that CO2 played a lesser role in influencing ice age temperatures than was thought. This suggests that the most extreme predictions for global warming from rising CO2 are less likely.

Cloudbase conundrum

In other recent research, scientists note that clouds are a major source of uncertainty in climate models. Some make the Earth warmer by trapping heat, others make it cooler by reflecting sunlight. Exactly which type of cloud influences climate is a major problem. In some cases it is unknown if they cause overall cooling or warming.

New remote-sensing measurements could provide a way to estimate droplet concentration in clouds, which will enable scientists to gain insights into how changes in atmospheric aerosol levels could affect clouds and climate.

They have made the first-ever remote observations of the fine-scale structure at the base of clouds. The results have just been published in Nature Climate and Atmospheric Science and reveal that the air-cloud interface is not a perfect boundary, but is a transition zone, where aerosol particles suspended in Earth’s atmosphere give rise to the droplets that form clouds. Most cloud droplets initially form at the cloud base, in the so-called ‘droplet activation zone’, and the number of them will affect the properties of the cloud later on, including how much sunlight it reflects.

To investigate this zone, scientists fired laser beams into the atmosphere and measured the backscattered reflection from molecules, aerosols, and cloud droplets. Recently, a new higher-resolution laser, firing at 20,000 pulses a second, has focused in on this important area. Cloud microphysical properties and processes are crucial to understanding weather and climate and this new technique may provide a valuable contribution.