USGCRP-sponsored research continues to advance understanding of the causes, magnitude, and consequences of climate change over decades to centuries. The FY96 edition of Our Changing Planet included a summary of recent key findings. Highlights of more recent findings follow:

• Measurements of global surface air temperature over the land and of sea surface temperature over the oceans indicate that 1995 was the warmest year since the beginning of continuous records circa 1860 (see figure). Despite the century's largest volcanic eruption in 1991, four of the warmest years of the 140-year record occurred during the 1990s. Proxy records from ice cores and other long-term data archives indicate that this century is the warmest in recent history. This evidence that temperatures are rising is further confirmed by indications of receding mountain glaciers and rising sea level.

• Satellite data from the Stratospheric Aerosol and Gas Experiment II (SAGE II) instrument indicate that the very substantial stratospheric aerosol layer caused by the June 1991 volcanic eruption of Mt. Pinatubo had been significantly thinned by early 1994, but that aerosol levels were still nearly 10 times greater than before the eruption, exerting a cooling influence that lasted several years. Data from the Microwave Sounding Unit (MSU) also showed that the stratosphere was warmed and the troposphere was cooled by the Pinatubo eruption, thereby requiring a correction for these events before performing a trend analysis with this data set in search of human-induced climatic change.

• Tropospheric aerosols, both from sulfur dioxide emissions from fossil fuel combustion and from biomass burning, have been demonstrated to have had a regional cooling influence on the climate. Model simulations suggest that, at least near and downwind of source regions, aerosol-induced cooling could be offsetting greenhouse-induced warming, although the patterns of change and the subsequent response of the global climate are complex. Inclusion of the cooling influence of aerosols in model simulations of climate change has improved the agreement between simulated and observed temperature changes over the last century.

• Recent improvements in the ability to compare the spatial and temporal patterns of climate change predicted by models with observations suggest that the geographic pattern of climate change is more likely due to emissions of greenhouse gases and aerosols than to natural variations in the climate. Similarly, the latitudinal and latitudinal pattern of change is more likely to be due to human-induced changes than to changes in natural factors such as volcanic eruptions and solar radiation. Together, these global and regional analyses suggest that a significant fraction of climate change over the last century is due to human rather than natural influences.