Climate Science: Climate Change and Its Impacts

Policy Reports | Energy and Natural Resources | Global Warming

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


In 1988, James Hansen, director of NASA's Goddard Institute of Space Studies, testified before the United States Senate that, based on computer models and temperature measurements, he was "99 percent" certain "... the [human-caused] greenhouse effect has been detected and it is changing our climate now."1 His statement was widely covered by the media and first brought the term "global warming" to the general public's attention. Many of his colleagues thought his announcement was premature at best and rash at worst, including two scientists who testified at the same hearing.2 Since then, climate science has improved but 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.

"Global warming began long before industrial development led to increases in greenhouse gases."

The Earth's climate began a recent general warming trend long before global industrial development led to substantial increases in greenhouse gases beginning in the middle of the 20th century. Indeed, the Earth's climate has been warming since the "Little Ice Age" ended in the mid-1800s. [See Figure I.] The period from about 1500 to the mid-1800s is known as the Little Ice Age because glaciers around the world grew to their greatest extent since the Northern Hemisphere ice age ended about 20,000 years ago. Because the Earth's climate began warming before substantial amounts of greenhouse gases were added to the Earth's atmosphere in the middle of the 20th century, natural variability accounts for all or nearly all of the warming prior to the 1940s — when approximately half of the observed 20th century warming occurred and before significant human-caused increases in greenhouse gases began. Similarly, natural variability must be considered as a possible cause of warming since the 1940s until and unless scientific evidence proves otherwise.

Figure I - Climate Cycle

However, based on the assumption that climate change is occurring and that human activities — principally emissions of greenhouse gases, such as carbon dioxide (CO2) — are major contributors to global warming, the United Nations established its Intergovernmental Panel on Climate Change (IPCC) in 1988. Since then, the IPCC has issued First, Second and Third Assessment Reports (1990, 1996 and 2001, respectively) as well as a number of special reports. Additionally, there have been regional studies, including the United States National Assessment of the Potential Consequences of Climate Variability and Change (USNA or National Assessment). These reports profess to represent the state of climate science. But claims that these reports represent a scientific consensus that human activities are causing or will soon cause significant and generally harmful global warming ignore uncertainties noted in the reports themselves, internal inconsistencies in the supporting data, inconsistencies among the various climate models, and many studies that have reached contrary conclusions.

"The Earth's climate has been warming since the mid-1800s."

Global warming alarmists assert that greenhouse gas emissions will cause significant changes in global temperature, storm frequency, rainfall and species extinction. However, scientific evidence undermines these assertions. Unfortunately, extreme predictions reported in the popular press adversely affect the public's perception of the real science behind global warming.

This study examines the state of climate science and our understanding of climate change. The underlying theme is that the climate system is complex, resilient and inherently stable. Research from a wide variety of disciplines, published in a number of peer-reviewed journals and spanning a broad spectrum of climate expertise, supports this conclusion. Throughout the millennia, life has survived and thrived despite wide changes in solar output, air temperature and atmospheric gas concentrations.

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