Climate Science: Climate Change and Its Impacts

Studies | Environment | Global Warming

No. 285
Monday, May 15, 2006
by David R. Legates, Ph.D., C.C.M.

Executive Summary

Scientific debate continues regarding the extent to which human activities contribute to global warming and what the potential impact on the environment might be. Importantly, much of the scientific evidence contradicts assertions that substantial global warming is likely to occur soon and that the predicted warming will harm the Earth's biosphere.

The Earth's climate began a warming trend after the "Little Ice Age" ended in the mid-1800s, long before global industrial development led to substantial increases in greenhouse gases beginning in the middle of the 20th century. About half of the warming during the 20th century occurred prior to the 1940s, and natural variability accounts for all or nearly all of the warming.

To assess future climate trends, climatologists rely upon General Circulation Models (GCMs) that attempt to describe Earth's climate. The many climate models in use vary widely with respect to the variables they include and in the assumptions they make about how those variables interact. Yet some official reports, including the U.S. National Assessment published in 2000, report only the most extreme predictions, ignoring others that project only moderate warming in the 21st century.

Global warming alarmists have attributed increases in hurricanes, floods, droughts, tornadoes, hail storms and heat waves to global warming caused by human activities. However, the evidence does not support their claims. In recent months, for instance:

  • The unprecedented destruction caused by Hurricanes Katrina and Rita was blamed on climate change — but experts say recent, more powerful storms are part of a natural cycle, and greater hurricane damage in North America is due to increased coastal populations and development rather than more severe storms.
  • Similar claims have been made about other weather phenomena in North America ; but, in fact, there is no evidence of an increase in the frequency or severity of floods, droughts, tropical cyclones, tornadoes, hail storms or other severe weather events.

Some have attempted to link the present warming trend to secondary effects, such as species extinction. However, the relationship between species extinction and climate change is even more tenuous. For example:

  • Recent claims that polar bear populations are threatened by global warming ignore the fact that only two polar bear populations are declining, others are increasing in numbers and the majority have stable populations.
  • Recent claims that coral reefs are "bleaching" (losing color and dying off) due to warming oceans ignore the evidence that bleaching appears to be a healthy response in which corals expel one symbiotic species of algae for a better-adapted species that allows corals to thrive in warmer waters.

It has also been claimed that low-lying coastal areas are endangered due to rises in sea level as the Arctic pack ice, glaciers and the mile-thick Greenland Ice Sheet melt in a warming climate. However, the evidence does not show this is occurring:

  • The fact that parts of the Arctic Ocean are ice-free in the summer is said to be evidence that sea ice and the pack ice along the Arctic coast are disappearing; but changing wind patterns pushing the ice around, not rising temperatures, are responsible for navigable Arctic waters.
  • In Alaska, home to many glaciers, several decades of increasingly colder temperatures in the middle of the 20th century preceded a more recent return to the average temperatures of the early 20th century.
  • Temperatures at the peak of the Greenland Ice Sheet show it is actually growing colder.
  • Sea levels have been rising — in fact, they have been rising since the end of the last ice age 20,000 years ago — but there is no evidence of an accelerating trend.

The complexity of the climate and the limitations of data and computer models mean projections of future climate change are unreliable at best. In sum, the science does not support claims of drastic increases in global temperatures over the 21st century, nor does it support claims of human influence on weather events and other secondary effects of climate change.

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