State Senate workshop on climate change

State Senate workshop on climate change
The evidence for climate change

Philip Mote
March 26, 1999
Our presentation today rests on three foundational premises, and it therefore makes sense to discuss those premises before proceeding. First, carbon dioxide keeps the planet warm. Second, the amount of carbon dioxide in the atmosphere will increase indefinitely -- for at least a hundred years -- mainly because we humans are burning fossil fuels. Third, increasing the amount of carbon dioxide in the atmosphere will change the climate. From these premises, it follows that global climate change is already underway and will almost certainly continue indefinitely: for at least 100 years. This was, in essence, the conclusion of a comprehensive international study on climate change by the Intergovernmental Panel on Climate Change, or IPCC. In its 1995 report the IPCC said for the first time that ``The balance of evidence suggests a discernible human influence on global climate.'' They also projected an increase in globally averaged temperature during the next century of about 4oF.

Because these conclusions have sparked controversy and much bad press, it is important to address any doubts you may have about their validity. Even the few scientists who dispute the evidence that humans are changing climate agree with points (1) and (2). The dispute over point (3) centers on two issues, both concerning the record of global average temperatures provided by surface thermometers (Figure 1). This record shows an increase in the early part of the century, a level period in the middle of the century, and a sharper increase since about 1970. The two questions about this record are: First, is it representative of the true global average temperature, or are the thermometers too far apart and too contaminated by urban growth? Second, can the observed warming be explained by natural phenomena?

Urban growth undoubtedly contaminates the trends shown, but many thermometers unaffected by urban growth show similar trends. In fact, the largest trends observed are in the Arctic, which is not an area of rapid urban growth. Scientists have estimated that the impact of urban warming on the global trends is small. Scientists have also estimated that with few exceptions, the patterns of trends are so much larger than the distance between stations that the network we have is certainly adequate to capture the trends. The exception is over the oceans in the southern hemisphere.

Given the huge gaps between observing stations in the southern oceans, and the possible contamination by urban warming, wouldn't it be better to use satellite measurements? Since 1979, satellites have measured temperatures, over nearly the whole earth, of a thick layer of the upper atmosphere roughly around the altitude at which airplanes fly. The satellite record (Figure 2) showed almost no trend for 1979-1997, and furthermore, it agreed very well with the trend calculated from balloon measurements. You might be wondering, why does the upper atmosphere matter? No one lives there. The reason it matters is that the climate models say that this layer of the atmosphere should be warming at least as fast as the surface. Thus we have a discrepancy between the climate models and the observations. Are the models wrong? Is climate change a hoax?

Another explanation for this discrepancy is not that models are wrong about the whole issue of climate change but that they are wrong about the connection between the surface and the upper atmosphere. Look what happened just before the satellite measurements began: a fairly abrupt warming of the upper atmosphere. The upper atmosphere appears to warm in fits and starts instead of smoothly. Another jump may have occurred in 1998.

It's not just thermometers that indicate that the world's surface temperature is rising. Most of the world's glaciers are melting back, and temperatures measured at various depths in the soil also indicate that recent years are warmer than earlier years. Using these and countless other indicators of temperature, like the growth rings of ancient trees, some scientists have pieced together a record of climate over the last 1000 years (Figure 3) which shows that the warming in this century is unprecedented in the last millenium.

The overwhelming evidence, then, supports the view that climate is changing: the Earth is warming. But is this warming natural?

Scientists have compared human influence on climate with the influence of natural phenomena, primarily small variations in solar output and the huge dust clouds belched high into the atmosphere by the occasional volcano. Before about 1970, in the contest to control climate nature was the major player, but since then humans have taken the lead (Figure 4). In 1970, the CO2 concentration in the atmosphere was only 17% above natural levels. As CO2 concentrations have risen since then, the temperature has risen as well. Today, with CO2 concentrations at more than 30% above natural levels and projected to rise ever faster, the doubts about climate change are dwindling.



--------------------------------------------------------------------------------