Near the end of December 1999, scientists at the National Climatic Data Center announced that the decade of the 90’s was the warmest on record. During this decade, we have had one of the largest El Niņo events in history, which significantly warmed the atmosphere for over a year. Since that El Niņo ended, we have had a major La Niņa event, which has contributed to cooling of the atmosphere. In the early 90’s we also experienced a major volcanic eruption, Mt. Pinatubo, which significantly cooled the atmosphere for a year or two. Averaging over all of this, the 90’s were the warmest decade.

So what’s going on? How does one understand this? Where does global warming fit in all this? It is important to keep clear in our minds that climate varies naturally, and it is also likely to be changing due to human activities. For the foreseeable future, we expect natural climate variability to continue similarly to that of the past. There will be future volcanic eruptions that will contribute to cooling. There will continue to be El Niņo’s and La Niņa’s that alternately heat and cool the atmosphere. Beneath this variability there has also been a trend of increasing temperature growing steadily. Until recently, it has been hard to detect this signal.

In 1995, a group of climate experts produced a report (Climate Change 1995, The Science of Climate Change) in which they claimed to see a discernable human imprint on temperature changes, although the signal was weak. Within the next year, the next report in this series will be issued, and it is likely that another group of experts will make stronger claims that human activity is modifying the climate. I expect that the forthcoming claims will be controversial, just as were the claims made in 1995. However, I expect that by 2005, or 2010 at the latest, there will be general agreement, if not unanimity, that human activity is modifying the climate, and the questions will shift to how big is the expected change, where will it occur, and on what parts of the environment.

Research in the Climate and Global Dynamics (CGD) Division is focused on major problems that will help answer these questions. Over the past two years, we have carried out many simulations using the Climate System Model to investigate the effects of emission of greenhouse gases. Our results agree with those of other major modeling centers. We are entering an era of human-induced global warming. We recognize, however, that our present model, like models at other centers, is imperfect. Although clearly more realistic than our previous models, the Climate System Model has features that need to be improved or corrected. These model blemishes compromise, to some degree, the credibility of our results. Therefore, we are working on an improved version of the Climate System Model that produces simulations that look more like observations.

In addition to model improvements, we are expanding the capabilities of the model. We are expanding the model into the middle atmosphere; we are adding atmospheric chemistry and biogeochemistry components to the model. We will soon be able to address new aspects of climate change, such as how does solar variability affect climate, how has land surface changes due to human activity changed climate, and where does the carbon emitted by human activity go? How much goes into the ocean, into the land surface, and how much stays in the atmosphere? How do we expect ecosystems to change as a result of climate change?