Q: How can scientists tell that climate change is due to greenhouse gases, rather than part of a natural climate cycle?

A: The Earth's climate fluctuates naturally because of wobbles in the axis of rotation and variations in sunspot activity. For about 11E3 years we have been in a period of warming. Prior to this glaciers covered much of the Northern Hemisphere and the temperature was about 5 degrees cooler. Even in the last 2000 years there has been a variation in average temperatures if 1 to 1.5ºC. Northern Hemisphere thermometer records, as well as climate data from tree rings and ice cores has shown that the 20th century was unusually warm compared to the last 1000 years. Other evidence includes increased heatwaves, fewer frosts, warming of the oceans and lower atmosphere, less snow and retreat of glaciers and sea ice. Sea levels have increased 10 to 20cm. Climate scientists have run computer simulations to see how the observed warming fits in with the natural climate variability and human-induced climate change. Natural climate variability from solar irradiance and volcanic eruptions explains much of the warming of the first half but not the second half of the 20th century. When simulations take into account both greenhouse gases and aerosols (small airborne particles) due to human activities, the simulated warming is consistent with that observed in the second half of the 20th century. Scientists have concluded that most of the global warming of the last 50 years is due to human activities. Climate models driven by scenarios of greenhouse gas and aerosol (small particle) emissions estimate a rise in temperature of 1.4 to 5.8ºC by 2100 assuming no actions to reduce climate change. While this is comparable to previously occurring climate change, it is occurring at an unprecedented pace and the changes would affect a larger and less mobile population.

Hennessy K.; 2005; Climate Impacts Group; CSIRO Atmospheric Research; in Climate change is one of the major challenges facing humanity in the 21st Century. We asked for your questions on climate change and selected the most frequently asked and the most intriguing ones. Here are the answers from some of Australia's top climate change experts.; the Australian Broadcasting Corporation's online gateway to science; ABC Science Online, Sydney - AUS;
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