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Earth’s climate has changed over the past century. The atmosphere and oceans have warmed, sea levels have risen, and glaciers and ice sheets have decreased in size. The best available evidence indicates that greenhouse gas emissions from human activities are the main cause. Continuing increases in greenhouse gases will produce further warming and other changes in Earth’s physical environment and ecosystems.

Global climate has varied greatly throughout Earth’s history. In the final decades of the 20th century, the world experienced a rate of warming that is unprecedented for thousands of years, as far as we can tell from the available evidence. Global average temperature rise has been accompanied by ongoing rises in ocean temperatures, ocean heat storage, sea levels and atmospheric water vapour. There has also been shrinkage in the size of ice sheets and most glaciers. The recent slowdown in the rate of surface warming is mainly due to climate variability that has redistributed heat in the ocean, causing warming at depth and cooling of surface waters. Australia’s climate has warmed along with the global average warming.

If greenhouse gas emissions continue to grow rapidly, it is expected that, by 2100, the global average air temperature over the Earth’s surface will warm by around 4°C above mid-19th century temperatures. There are many likely ramifications of this warming. However, if emissions are reduced sufficiently rapidly, there is a chance that global average warming will not exceed 2°C and other impacts will be limited.

Sea levels have risen during the 20th century. The two major contributing factors are the expansion of sea water as it warms, and the loss of ice from glaciers. Sea levels are very likely to rise more quickly during the 21st century than the 20th century, and will continue to rise for many centuries.

There is near-unanimous agreement among climate scientists that human-caused global warming is real. However, future climate change and its effects are hard to predict accurately or in detail, especially at regional and local levels. Many factors prevent more accurate predictions, and some uncertainty is likely to remain for considerable time. Uncertainty in climate science is no greater than in other areas where policy decisions are routinely taken to minimise risk. Also, the uncertainty means that the magnitude of future climate change could be either greater or less than present-day best estimates.

‘The science of climate change: Questions and answers’ was prepared by a working group of nine members co-chaired by Dr Ian Allison FAA and Professor Mike Raupach FAA FTSE. The document was also reviewed by an oversight committee of eight members chaired by Professor John Zillman AO FAA FTSE. This publication is an update of the Academy’s 2010 booklet of the same name.