Scientists find subtle wind variations may spur Abrupt Climate Change

German and Spanish climate scientists have found that subtle changes in wind strength can significantly influence the global climate and may even have been responsible for Abrupt Climate Change in the past. The findings, published in Geophysical Research Letters, shed light on the dynamics of the Atlantic meridional overturning circulation, which is the oceans' heat engine.

Abrupt Climate Change (ACC) refers to an event where a large and widespread shift in climate occurs within a short period, in time spans as short as a decade. Most of the current studies and debates on potential climate change have focused on the ongoing buildup of industrial greenhouse gases in the atmosphere and a gradual increase in global temperatures. But recent and rapidly advancing evidence demonstrates that Earth's climate repeatedly has shifted dramatically and suddenly in the past. It is conceivable that human forcing of climate change is increasing the probability of large, abrupt climate events. Analysis of past ACC events can help today's research into the eventuality of a repetition of such an event, this time induced by human activities.

Simulating the climate during the Last Glacial Maximum (LGM), which occurred roughly 21,000 years ago, is a major challenge for climate modeling, say Marisa Montoya of the Department of Astrophysics and Atmospheric Sciences at the Universidad Complutense de Madrid, and Anders Levermann of Earth System Analysis at the Potsdam Institute for Climate Impact Research, Germany. In particular, the Atlantic meridional overturning circulation (AMOC) - also known as the Thermohaline Circulation (THC) or sometimes called the ocean conveyor belt (see image, click to enlarge)- which regulates climate by distributing heat to the world's oceans and involves deepwater formation in the North Atlantic, is poorly constrained in model scenarios.

To characterize the AMOC during the LGM, models must accurately simulate surface winds, which facilitate horizontal and vertical mixing in the ocean. Noting that wind fields during the LGM are not well understood, Montoya and Levermann model how changes in wind strength would affect AMOC strength.

By assuming that LGM wind stresses are proportional to those experienced today, the authors discover that below certain thresholds of wind strength, North Atlantic deepwater formation takes place south of Greenland and the AMOC is relatively weak. Above this threshold, deepwater formation occurs farther north, leading to a vigorous AMOC. This suggests that subtle wind variations can significantly influence climate, perhaps even spurring abrupt climate change events:
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Were such an event to recur, the economic and ecological impacts could be large and potentially serious. Unpredictability exhibited near climate thresholds in simple models shows that some uncertainty will always be associated with projections. In light of these uncertainties, scientists have urged policy-makers to consider expanding research into ACC, improving monitoring systems, and taking actions designed to enhance the adaptability and resilience of ecosystems and economies.

One research group that was given a mandate by the G8 to study ways to avert and adapt to ACC is the Abrupt Climate Change Strategy Group (ACCS).

The key strategy which it designed to prevent ACCS is the rapid implementation of carbon-negative bioenergy systems. All coal plants would be forced to switch to biomass, which is combusted as solid biofuels to power societies, while the CO2 released from this carbon neutral energy is sequestered in geological formations. This way, "negative emissions" are generated that can turn the tide and reverse climate change.

Writing about ACC and how bio-energy with carbon storage systems (BECS) can be implemented, Dr Peter Read and Jonathan Lermit of the ACCS indicate that:

Abrupt Climate Change (ACC - NAS, 2001) is an issue that ‘haunts the climate change problem’ (IPCC, 2001) but has been neglected by policy makers up to now, maybe for want of practicable measures for effective response, save for risky geo-engineering.

Such geo-engineering plans are circulating within the scientific community but they are very costly and present major risks. Ideas include launching mirrors into space or iron seeding oceans on a massive scale. A geo-engineering idea by Nobel-Prize winner Paul Crutzen consists of filling the upper atmosphere with sulphur, to emulate the climate cooling effects of volcanos. However, this idea was soon dismissed as too risky and deadly. A number of simulations show that virtually all geo-engineering methods proposed so far present large risks that could be deemed unacceptable.

The Abrupt Climate Change Strategy Group however identified bio-energy with carbon storage (BECS) as a safe, feasible, efficient and cost-effective intervention, that performs on the scale of geo-engineering options, but allows societies to function more or less as normal.

Negative emissions energy systems are key to responding to ACC because – taking account of rising levels on non-CO2 greenhouse gases, for which no means exists for accelerating natural removal processes – the need may be to get to CO2 levels below pre-industrial. This cannot be done by natural absorption, even with zero emissions energy [such as wind, solar, nuclear].

A portfolio of Bio-Energy with Carbon Storage (BECS) technologies, yielding negative emissions energy, may be seen as benign, low risk, geo-engineering that is the key to being prepared for ACC. The nature of sequential decisions, taken in response to the evolution of currently unknown events, is discussed. The impact of such decisions on land use change is related to a specific bio-energy conversion technology. The effects of a precautionary strategy, possibly leading to eventual land use change on a large scale, is modeled. - Read and Lermit, ACCS

The advantage of BECS is that it allows societies to function in a relatively normal manner, because this geo-engineering option does not affect energy supplies. Even more, it is the only strategy that produces energy while taking carbon dioxide out of the atmosphere (none of the other geo-engineering strategies yield energy during their implementation).

As scientists are more and more talking terms of radically cutting global CO2 emissions, such carbon-negative bioenergy systems have become the key to achieving this aim. Negative emissions energy systems can be implemented today, at relatively low cost and by using existing infrastructures (coal plants switching to biomass; natural gas plants swithing to biogas and synthetic biomass based gas - SNG).

References:
Montoya, M., and A. Levermann (2008), "Surface wind-stress threshold for glacial Atlantic overturning", Geophys. Res. Lett., 35, L03608, doi:10.1029/2007GL032560.

Woods Hole Oceanographic Institution: Abrupt Climate Change.

Abrupt Climate Change Strategy Group.

P. Read and J. R. Lermit, "Bio-energy with carbon storage (BECS): a sequential decision approach to the threat of abrupt climate change", Energy, November 2005, vol. 30, no14, pp. 2654-2671 [*pdf - link to full article located at ACCStrategy].

Biopact: Abrupt Climate Change and geo-engineering the planet with carbon-negative bioenergy - December 21, 2006