The “hockey stick” graph featured in the 2001 IPCC assessment, showing temperature (departures from the 1961–1990 average) in the Northern Hemisphere during the past 1000 years. (IPCC)

Earth’s orbit around the sun is not precisely circular, but elliptical, with the sun positioned slightly to one side of the center point. Currently, this brings Earth about 3% closer to the sun in early January (perihelion) than in early July (aphelion), with about 7% more solar energy reaching Earth at perihelion. The eccentricity, or “off-centeredness,” of the orbit varies over time in a complicated way. The net result is two main cycles, one that averages about 100,000 years and another that runs about 400,000 years. When the eccentricity is low, there’s little change through the year in the Earth–sun distance. When eccentricity is high, the sunlight reaching Earth can be more than 20% stronger at perihelion than at aphelion. This either intensifies or counteracts seasonality, depending on the hemisphere and the time of year that perihelion and aphelion occur. The last eight ice ages have come and gone roughly in sync with the 100,000-year eccentricity cycle, but it’s not yet clear how strongly the two are related (see p. 296).

Over billions of years. On this scale, the details are blurry, but the overall picture is clear. The sun will continue to heat up and eventually undergo changes to its size and structure, producing a climate on Earth hot enough to evaporate the oceans and make life impossible. Eventually, the sun will shrink and start to fade, with the solar system reaching its cold “final configuration” in around 12 billion years.

James Croll. (J.C. Irons/photo courtesy NOAA)

Over tens of millions of years. A gradual, though sometimes erratic, cooling trend has been under way for at least 55 million years, perhaps due in part to carbon dioxide removed by weathering on top of the growing Himalayas. Northern Hemisphere glaciations began about 2.7 million years ago. Warm and cool periods come and go on top of this overall cooling. Simple extrapolation would keep Earth on this extremely slow cooling trend, although there's no evidence to tell us how long that might last.

Over tens of thousands of years. The most recent ice age began about 115,000 years ago and ended about 11,500 years ago. Then came a dramatic warm-up, which lasted until about 3000 BC. Since then, Earth's temperature has changed relatively little, with a very slight cooling interrupted by warmer periods and punctuated by the last century's sharp temperature rise. More than a thousand years from now, after humans have exhausted (or stopped burning) fossil fuels and much of the resulting greenhouse gas has left the atmosphere through natural processes (mostly through slow absorption by the ocean), we may return to cooler times. If the length of the last interglacial is any guide, we're due for another ice age in the next few thousand years, although some scientists believe it's more likely to take 20,000 years or more. It's even possible that a mix of polar melting, land-use changes, and slowly waning greenhouse gas concentrations could postpone the next ice age for an undetermined period.

Over hundreds of years. Around AD 1000, Earth was nearly as warm as it is now. After that, temperatures over much of the planet cooled by around 0.2°–0.5°C (0.4°–0.8°F), mostly during the period known as the Little Ice Age, from the 1400s to the mid-1800s. The global average has since rebounded by roughly 1°C (1.8°F) as human-produced greenhouse gases have increased. According to the 2013 IPCC report, further warming this century could run anywhere from 0.3° to 4.8°C (0.5°–8.6°F), depending on the rate of emissions. Further warming after 2100 could be substantial if greenhouse emissions continue to increase through this century, and the delayed response of ice sheets and deep oceans to warming could produce major sea level rise over several centuries.

Dutch painter Hendrick Avercamp (1585–1634) was a specialist in winter scenes, especially skaters on frozen rivers, canals, and flood waters. (Hendrick Avercamp [public domain], via Wikimedia Commons)

Scientists take ice core samples in cold, windy, and forbidding environments. (Ted Scambos and Rob Bauer, U.S. National Snow and Ice Data Center)

The Indian subcontinent about to crash into Asia 60 million years ago.

The process involved in undersea volcanism and the surface and atmospheric effects.

James Lovelock. (Bruno Comby/Wikimedia Commons)

An ancient bristlecone pine from the White Mountains of California. (Gnarly/Wikimedia Commons)