This is an extract from The Science of Climate Change — Questions and Answers, published by the Australian Academy of Science and distributed to members of parliament, every local government authority in Australia and every Australian high school, in August 2010. Crikey will be running a series of extracts, including canvassing common myths.
Climate has varied enormously through Earth’s history
Since the Earth was formed 4.5 billion years ago, the global climate has changed dramatically many times due to the changing configuration of continents and oceans, natural variations in the levels of greenhouse gases in the atmosphere, the sun’s intensity, and the Earth’s orbit around the sun.
Evidence from the past shows that global climate is sensitive to small influences
During the past million years, the average temperature of the Earth’s surface has risen and fallen by about 5°C, through 10 major ice age cycles. The last 8000 years have been relatively stable at the warmer end of this temperature range. These cycles were initiated by subtle variations in the Earth’s orbit that altered the pattern of absorbed sunlight. Measurements from ice cores and other sources strongly suggest that as temperatures changed, other changes were triggered that had an amplifying effect: during warm periods, CO2 and methane were released into the atmosphere, and ice sheets receded and so reflected less sunlight to space. This meant that small influences were amplified into larger changes.
An important implication of this finding from past climate changes is that similar processes are likely to amplify current human influences on climate. Past temperature changes affected the world dramatically. For example, in the coldest period of the last ice age (approximately 20,000 years ago) sea level was at least 120 metres lower.
The atmosphere was also very dusty, probably because of dramatic regional reductions in vegetation cover associated with the colder climate and reduced CO2. In even earlier times, several million years ago, global temperature was several degrees higher than today and warm, tropical oceans may have reached much farther from the equator, causing significant changes to atmospheric flow patterns.
Records also show that climates can shift abruptly
The largest global temperature changes evident in the geologic record have typically occurred fairly slowly over tens of thousands or millions of years, much more gradually than the warming over the past century. However, some rapid changes have been documented in very warm past climates and in more recent ice ages. One of these rapid changes took place 56 million years ago, when the global temperature increased by about 5°C, accompanied by an unexplained release of greenhouse gases into the atmosphere.
This release may have been so rapid as to be comparable to the current human release of fossil fuels. Other rapid changes during the last ice age, of 5°C or more over as little as a few decades, were probably mostly regional and due to sudden collapses of ice sheets or changes in ocean currents.
Although the millennium before the industrial revolution was relatively stable, there were variations in climate over that period.
The Medieval Warm Period (AD 800-1300) and Little Ice Age (AD 1500-1800) are two well-known climate episodes during the past thousand years. The Northern Hemisphere may have been up to 1°C warmer on average during the former period than during the latter. However, several assessments indicate that Northern Hemisphere average temperatures over the past 50 years have been warmer than during the Medieval Warm Period, and temperatures over the past decade are warmer still. Records are sparse in the Southern Hemisphere, but those available indicate little or no correlation with warming in the Northern Hemisphere during the Medieval Warm Period, unlike the more globally coherent cooling in the Little Ice Age and warming over the past century.
There have also been regional variations in climate, particularly rainfall, that are not associated with global changes. For example, regional droughts appear to have contributed to the collapse of the ancient Akkadian empire in the Middle East and the Mayans in Mexico.
The Australian Academy of Science, which represents Australia’s foremost scientists, provides scientific advice to policy makers and promotes excellence in Australian science, has devoted considerable resources to untangling the science of climate change and presenting it in a simple and easily understood format.
The full report can be downloaded here for free.