Detailed analyses of ocean sediments, ice cores, and other data show that for at least the last 2.6 million years, Earth has gone through extended periods when temperatures were much lower than today and thick blankets of ice covered large areas of the Northern Hemisphere. These long cold spells, lasting in the most recent cycles for around 100,000 years, were interrupted by shorter warm ‘interglacial’ periods, including the past 10,000 years.
Through a combination of theory, observations, and modelling, scientists have deduced that the ice ages* are triggered by recurring variations in Earth’s orbit that primarily alter the regional and seasonal distribution of solar energy reaching Earth. These relatively small changes in solar energy are reinforced over thousands of years by gradual changes in Earth’s ice cover (the cryosphere), especially over the Northern Hemisphere, and in atmospheric composition, eventually leading to large changes in global temperature. The average global temperature change during an ice-age cycle is estimated as 5 °C ± 1 °C (9 °F ± 2 °F).
*Note that in geological terms Earth has been in an ice age ever since the Antarctic Ice Sheet last formed about 36 million years ago. However, in this document we have used the term in its more colloquial usage indicating the regular occurrence of extensive ice sheets over North America and northern Eurasia.
In addition to emitting greenhouse gases, human activities have also altered Earth’s energy balance through, for example:
The future evolution of Earth’s climate as it responds to the present rapid rate of increasing atmospheric CO2 has no precise analogues in the past, nor can it be properly
understood through laboratory experiments. As we are also unable to carry out deliberate controlled experiments on Earth itself, computer models are among the most important tools used to study Earth’s climate system.
Climate models are based on mathematical equations that represent the best understanding of the basic laws of physics, chemistry, and biology that govern the behaviour of the atmosphere, ocean, land surface, ice, and other parts of the climate system, as well as the interactions among them. The most comprehensive climate models, Earth-System Models, are designed to simulate Earth’s climate system with as much detail as is permitted by our understanding and by available supercomputers.
The capability of climate models has improved steadily since the 1960s. Using physics-based equations, the models can be tested and are successful in simulating a broad range of weather and climate variations, for example from individual storms, jet stream meanders, El Niño events, and the climate of the last century. Their projections of the most prominent features of the long-term human-induced climate change signal have remained robust, as generations of increasingly complex models yield richer details of the change. They are also used to perform experiments to isolate specific causes of climate change and to explore the consequences of different scenarios of future greenhouse gas emissions and other influences on climate.