Overview

Much of the recent scientific research on human-induced climate change has assumed that atmospheric carbon dioxide concentrations will double over the next century (2xCO2), resulting in a globally averaged temperature increase of 1 to 3.5�C by the year 2100. However, analyses of future emission scenarios in the 1995 Intergovernmental Panel on Climate Change (IPCC) assessment report and elsewhere indicate that, on a multi-century time scale, if the build-up of greenhouse gas emissions continues in a business-as-usual manner, CO2 levels would rise to well beyond a doubling unless very substantial reductions occur in the projected emissions. In fact, the median emissions scenario (IS92a) used in the 1995 IPCC assessment report suggests that we are already on a trajectory toward a quadrupling of the concentration of CO2 in the atmosphere. Prudence would, therefore, dictate that science explore the implications of the long-term build-up of atmospheric CO2 and, in the process, attempt to better understand what might constitute "dangerous anthropogenic interference" with the Earth's climate system. In this seminar, Dr. Mahlman will examine the potential impacts of quadrupling the concentration of CO2 in the atmosphere (4xCO2), and compare those results to those associated with a projected doubling of the concentration of atmospheric CO2.

Model Projections of Climate Change in a Quadrupled CO2 World

On a multi-century time scale, the cooling influence due to the contribution from anthropogenic sulfate aerosol pollution will be overwhelmed by the importance of the much longer lived increase in the CO2 concentration. Aerosols will, in time, become largely irrelevant in terms of their impact on global climate change. Warming from the increasing concentrations of CO2 is projected to be especially large over much of the mid-latitude continental regions, including North America and Asia. Model projections of temperature changes in such a future world are as much as 10 to 14�C higher than today. Sea-ice coverage over the Arctic Ocean is projected to decrease substantially; during late summer, for example, sea ice is virtually absent in the 4xCO2 world projection.

The process of climate change is very long-term. Because the heat-driven expansion of seawater as the oceans warm will only slowly heat the entire ocean, the thermally driven rise in sea level is expected to continue for centuries after the "model" build-up of atmospheric CO2 ceases. In the 4xCO2 projection, for example, sea level continues to rise steadily well beyond 500 years into the future, even though the build-up in CO2 ceases after 140 years. At the 500-year point in the future, sea level is projected to have risen by approximately 2 meters (roughly 6 feet). It is also quite possible that the effect of melting of continental ice sheets could increase these numbers substantially.

Model simulations also project that the global ocean's overturning circulation (oceanic circulation is presently responsible for transporting heat from the tropics to higher latitudes) would decrease sharply in intensity as greenhouse gas warming escalates. This would occur because the increased precipitation and runoff from the continents in high latitudes freshens high-latitude waters. In the 4xCO2 model experiment, the oceanic, salinity-driven circulation (thermohaline circulation) essentially ceases after 200 years. This change provokes a major change in the transport of heat by the oceans. In the 2xCO2 world experiment, the thermohaline circulation drops off significantly within 150 to 200 years' time, then recovers to its initial strength after several centuries. Model experiments show that the faster the rate of build-up of CO2, the larger the reduction in oceanic thermohaline circulation and the longer the delay in its recovery.

The GFDL model simulations also project significant decreases in soil moisture over most mid-latitude continental areas during the summer months. Typical model reductions in soil moisture over North America are on the order of 50% in the 4xCO2 experiment. Such changes would have a large impact on food production.

Model projections further reveal that as the concentration of atmospheric CO2 increases, both surface air temperature and atmospheric moisture content increase, resulting in an elevated heat index. The heat index (apparent temperature) is a measure of the stress imposed on humans and other species by elevated levels of atmospheric moisture and temperature. The higher the heat index, the harder it is for the body to dissipate heat. The elevation in the heat index for a 4xCO2 world is roughly double what it would be in a world with a doubled concentration of atmospheric CO2.

Possible changes to tropical storm intensity and/or frequency are presently uncertain but are also of great concern.