What Does the Science Say?

This is the thirtieth of a continuing series of essays and interviews from Earth Institute scientists on the prospects for a global climate-change treaty. Check with us daily for news and perspectives, and to make comments, as events unfold throughout the Copenhagen meetings.

The negotiations in Copenhagen have been handled by politicians and policy makers. But there would be no climate negotiations if climate scientists had not identified evidence that humans could disrupt the natural carbon cycle, and affect the climate system. The fact that some 50,000 people and the heads of most nations have come together indicates that, if nothing else, scientists have been quite successful in engaging society.

The overarching goal of the UN Framework Convention on Climate Change is to prevent dangerous interference with earth’s climate system. The original focus was on reducing the amount of greenhouse gases we emit, but in recent years, negotiators have focused increasingly also on minimizing the impacts of climate change by adapting infrastructure and economies. The disciplines underlying these goals range from atmospheric science and oceanography to ecology and engineering; and for anything to be realistically done, we will also need the fields of economics, behavioral sciences, law and political science, to name just a few.

Two areas of physical science are key to the discussions: carbon cycle science and climate science. Each of these is in itself interdisciplinary and overlapping. Carbon cycle science explores the many transformations of the element carbon and its movement among the atmosphere, rocks, soils, the ocean, and biological systems such as forests. The study of climate, generally defined as the average weather at a given site over a period of 30 or so years, includes research into the role of the sun’s energy, the movements of atmosphere and oceans, and the exchange of heat and moisture among the atmosphere and earth’s land, ocean and ice surfaces.

The evolving information from these fields has been repeatedly summarized and vetted by the Intergovernmental Panel on Climate Change (IPCC), with the most recent of its four reports being released in 2007. The IPCC has concluded that unless we reduce carbon emissions, the consequences of ongoing human-induced climate change include not just increased temperatures, but shifts in precipitation patterns; increased droughts in the subtropics; extinction of species; spread of infectious diseases; and sea level rise. Since 2007, further evidence of changes in climate have been observed (for instance, new record lows in Arctic summer sea ice), and understanding has progressed on feedback mechanisms that can magnify the effects of carbon dioxide (such as the effects of soot). Just this month, the World Meteorological Organization released a report indicating that 2000-2009 may be the warmest decade on record. The estimate includes air temperatures over both land and the ocean surface; and while global air temperatures have stabilized since 1998, the oceans have continued to warm, which means that on balance, things are still getting hotter.

Both carbon cycle science and climate science have a lot to say with regards to how much we should curtail emissions to minimize future warming. The European Union and the G8 nations, along with over 100 other countries, have adopted the “guardrail” that global average temperature should not surpass a rise of 2 degrees C (3.6F) over 1990 levels. Although we cannot assert with certainty that 2 degrees C would represent significantly greater or lesser risk than 1.5C or 2.5C, 2C is widely considered the threshold of dangerous human interference.

A recent study in the Proceedings of the National Academy of Sciences updates the assessment of the risks associated with a 2C increase. It says that when temperatures surpass 1.5C to 2C, then 20-30% of known plant and animal species are at increased risk of extinction, along with whole ecosystems such as coral reefs or the western forests of North America. Droughts and associated wildfires, heat waves, extreme precipitation events, floods and intense tropical cyclones are projected to be more frequent above 1C. After 2C, the study projects increased coastal flooding, reductions in water supply, and increased health impacts. The risk of uncontrollable shifts, such as melting of polar ice sheets, becomes more likely beyond 2C.

There have been many efforts to design what are known as “stabilization pathways” to lower emissions to reach a “safe” atmospheric level of carbon dioxide. For example, 450 parts per million is often considered an upper limit. One recent study took a novel approach and estimated how much carbon dioxide could be emitted between 2000 and 2050 to stay below the 2C “guardrail.” The researchers found that if we continue with business as usual, we will have emitted enough carbon dioxide by 2027 for there to be a 25% probability of surpassing a 2-degree C increase. Part of the uncertainty in the calculation occurs because while we can have a reasonably good estimate of emissions associated with energy use (power plants, transportation, buildings) and a slightly lesser understanding of deforestation and other land use changes, the natural carbon cycle is complex and changing.

In a nutshell, some of the carbon that enters the atmosphere from land changes or fossil fuel use remains there; but some is taken up by land plants or by the ocean. So in order to quantify the impact of emissions, we need to understand what portion of emissions stay in the atmosphere. To make things complicated, the amount of carbon that goes into the ocean or land changes with time depending on whether one or more of various natural cycles (such as El Niño) is operating, whether the ocean circulation is changing, or whether there are other factors that we don’t entirely understand.

The latest report of the Global Carbon Project indicates that about 43% of total CO2 emissions in 2008 stayed in the air—an increase compared to past estimates. In 2008 the uptake of CO2 by land was greater than usual, while that of the ocean was lower than usual— probably related to cooler, wetter conditions in the tropics and increased transport of deep carbon-rich water to the surface in the equatorial Pacific. Another recent study, headed by an oceanographer from Columbia University’s Lamont-Doherty Earth Observatory, also indicates that the ocean is absorbing increasingly less carbon dioxide. This study attributes this in part to shifts in ocean chemistry, brought on by the increasing CO2 itself that the water is absorbing.

Advancing our understanding of these processes is key to designing a pathway for emissions reductions. It is integral to this, and future climate negotiations. What countries decide to do with that information is a matter of politics, ethics and socioeconomics—not physical science. A final agreement on specific actions is looking unlikely at this point—but science must continue to play a decisive role in providing best estimates, and thus allow us to move forward in avoiding dangerous human interference in the climate system.