As we've reported recently, climate scientists are continuing to develop and refine climate change models in order to predict the effects of greenhouse gas emissions. One aspect of these models that hasn't been explicitly tested is their ability to capture rapid, irreversible changes to the climate system. The author of a commentary in this week's edition of Nature Geoscience argues that current climate models (such as those used in the Intergovernmental Panel on Climate Change [IPCC] reports) fail to simulate abrupt changes we've seen in the past, and therefore may be unable to predict similar events in the future.

Abrupt changes are those where the response (such as an increase or decrease in average global temperatures) occurs much more rapidly than do changes in the conditions that triggered it (increased atmospheric concentration of greenhouse gases, influx of water into the ocean, etc.). Obviously, in order to plan mitigation or adaptation strategies, we need to be able to predict the occurrence and severity of such events.

The author of the piece is Paul Valdes, a climatologist at the University of Bristol, who considered four past cases of abrupt change that current models are unable to simulate. In the first two, the models can't capture the starting conditions—incorrect initial conditions mean inaccurate simulated results (garbage in, garbage out). In the second two cases, simulations require unrealistically large perturbations to cause the changes that actually occurred in the past. All four cases seem to indicate that current models are too stable compared to the relatively sensitive climate system of the real world.

The first event considered, the Palaeocene-Eocene Thermal Maximum (PETM), which we've reported on recently, was a rapid warming event (5°C in the tropics and up to 20°C in higher latitudes, all within thousands of years) that occurred 55.8 million years ago. The PETM was likely caused by a release of methane from underwater hydrates that led to a massive carbon injection to the atmosphere (similar in magnitude to current events, though at a much slower rate). It's the subject of much study, but current models cannot simulate the warm climate that immediately preceded the climate change.

The second event discussed was the relatively recent (9,000-5,500 years ago) transition of the Sahara from wet and vegetated location into a desert, which occurred over a period of decades to centuries. As with the PETM, the models cannot predict the starting conditions—in this case the greening that occurred prior to desertification—and therefore cannot predict the abrupt transition.

The third case was a series of 25 rapid temperature increases (up to 8°C over a few decades) that occurred over the past 120,000 years during the Dansgaard-Oeschger climate cycles. Each started from a globally cold state and, though not completely understood, was likely caused by an influx of freshwater into the North Atlantic. Simulations in this case are able to predict abrupt change based on realistic starting conditions, but they require an injection of water that lasts thousands of years longer than anyone considers realistic.

The final case considered was the collapse of the Atlantic Meridional Overturning Circulation (AMOC) between 120,000 and 12,000 years ago. The AMOC is driven by density differences between warm water that flows north above colder water flowing south. The collapse, which took place over the course of six cycles known as Heinrich events, was caused by large amounts of freshwater from glaciers interrupting the normal circulation. The AMOC plays an important role in the global climate system, and its interruption led to rapid cooling in the Northern Hemisphere (10°C over a matter of years).

As with the simulations of the Dansgaard-Oescher warming events, climate models are able to predict an AMOC collapse, but only with an influx of 1 sverdrup (Sv, or 1 million m3/s) to the North Atlantic Ocean—that's 10 times higher than what's estimated to have hit the Atlantic during the last actual Heinrich event. It is particularly important to be able to predict this phenomenon, because a circulation collapse due to glacier meltwater is considered a possible effect of current warming.

Although climate models have been accused of being overly sensitive to changes in greenhouse gasses, it seems that in some cases, the models are too stable, requiring larger perturbations to cause the actual changes seen in the past. Because of this, the author cautions, the models underestimate the possibility of rapid events and therefore may give us a false sense of security.

Nature Geoscience, 2011. DOI: 10.1038/ngeo1200 (About DOIs).