CLIMATE SCIENCE:

SUMMARY: Removing greenhouse gases from the atmosphere will be insufficient to immediately halt global warming, according to recent computer simulations. The longer they are in the atmosphere, the longer their long-term warming effects will persist, due to a greater amount of heat taken up deep within the ocean.

Accounts of recent climate change in the popular media tend to suggest that eliminating pollutants from the atmosphere will herald an end to global warming. Unfortunately, this outcome is unlikely; heat taken up and slowly released by the ocean, induced by atmospheric carbon dioxide, could easily lead to a warming one thousand years in duration.

Susan Solomon (National Oceanic and Atmospheric Administration, United States) and coworkers report even more bad news. Although carbon dioxide is responsible for most of the anthropogenic (human-caused) global warming to date, computer simulations suggest that other less-abundant greenhouse gases will also exert their warming effects long after they are gone from the atmosphere.

Peristent warming.

Assume for a moment that carbon dioxide, nitrous oxide, and methane emissions increase as predicted until 2050, at which point emissions stop entirely (the latter is unrealistic, but is useful for demonstrating the scientists' main points). Global surfaces temperatures are predicted to rise to nearly 2.5°C above preindustrial times by 2050.

After that, temperatures remain above baseline for centuries. After a relatively "quick" drop of 1°C over the course of three centuries, temperatures are still nearly 1.5°C above baseline in the year 3000 (i.e. essentially irreversible global warming).

Approximately 1.8°C of the above-baseline warming by 2050 is due to carbon dioxide. The remainder of the approximately 2.5°C temperature increase is largly due to methane, and nitrous oxide to a lesser yet significant extent.

The effects of nitrous oxide and methane will be noticeable for many years, exceeding their persistence in the atmosphere, analogously to the effects of carbon dioxide previously reported. For example, two-thirds of the warming effects of nitrous oxide will still be present 114 years after a halt to its emissions (the atmospheric persistence of nitrous oxide is 114 years).

Why is this the case? Put simply, the longer a greenhouse gas is in the atmosphere, the more heat is trapped deep within the ocean, which is more slowly released than heat stored near the ocean surface.

Implications.

What do these findings mean in practice? The longer we wait to decrease greenhouse gas emissions, the longer the global warming effects will be exerted.

It isn't sufficient to put off cleaning up greenhouse gas emissions to a later date. Rapid cleanup, and rapid development of alternative sources of energy, are needed today.

NOTE: The scientists report no sources of funding for their research.

Susan Solomon, John S. Daniel, Todd J. Sanford, Daniel M. Murphy, Gian-Kasper Plattner, Reto Knutti, & Pierre Friedlingstein (2010). Persistence of climate changes due to a range of greenhouse gases Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1006282107