A Final Word

The goal of this site, a "Paleo Perspective on Global Warming" is to explain what paleoclimatic data are, where they come from, and what the data contribute to the global warming debate. We also attempt to clear up some of the confusion surrounding the issues of global warming, greenhouse warming, the ozone hole, and related change in the Earth's climate system. When one reviews all the data, both from thermometers and paleotemperature proxies, it becomes clear that the Earth has warmed significantly over the last 140 years. Global warming has occurred. Multiple paleoclimatic studies indicate that recent years, the 1990s, and the 20th century are all the warmest, on a global basis, of at least the last 1000 years. The most recent paleoclimate data reinforce this conclusion using longer records, new proxies, new statistical techniques, and a broader geographic distribution of paleo data. There are, however, questions remaining concerning global warming. For instance, what caused the warming and what are the implications for the future? The answers to these questions are not simple. There is considerable debate centered on the cause of 20th century climate change. Few people contest the idea that some of the recent climate changes are likely due to natural processes, such as volcanic eruptions, changes in solar luminosity, and variations generated by natural interactions between parts of the climate system (for example, oceans and the atmosphere). There were significant climate changes before humans were around and there will be non-human causes of climate change in the future. Nevertheless, with each year, more and more climate scientists are coming to the conclusion that human activity is also causing the climate to change. First on the list of likely human influences is warming due to increasing concentrations of greenhouse gases in the atmosphere. Other human activities are thought to drive climate as well. As the ice-core data show, the increase in carbon dioxide is unprecedented and well outside the range of natural variations. The recent increase matches the increase calculated from the fossil fuel emissions. There is little doubt that these gases will contribute to global warming, and here too the paleo record provides invaluable evidence regarding how much temperature change accompanied changes in carbon dioxide over the past several hundred thousand years. However, there is uncertainty about some issues. For example, these questions remain to be answered with complete confidence: How much warming has occurred due to anthropogenic increases in atmospheric trace-gas levels? How much warming will occur in the future? What other changes will occur with future warming? Paleoclimatology offers help in answering each of these questions. Several of the paleoclimate studies reported in this web document have begun efforts to attribute past climate change to both natural and human causes, and to estimate how much of the current warming is due to humans (i.e., greenhouse warming). The best estimate is that about 50% of the observed global warming is due to greenhouse gas increases. The paleo record also tells us how much temperature change occurred in the past when carbon dioxide levels were different. Studies show that the 100 ppm reduction in carbon dioxide during the last glacial was accompanied by a 3°C cooling in the western tropical oceans. This amount of temperature change is consistent with the change predicted by numerical climate model simulations. Changes at higher latitudes were much larger and included the growth of large ice sheets. Other studies of the glacial world show that many aspects of climate were different when carbon dioxide was reduced, including lower sea level, lower snowlines, and altered patterns of circulation. Based on these studies we can predict that other aspects of the climate in addition to temperature are likely to change with future warming.

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