Earth’s climate has often been much warmer or much cooler than it is today. About 700 million years ago, the earth was almost entirely covered in ice during events known as “Snowball Earth.” Later, Earth was entirely ice-free for hundreds of millions of years. Ice began to grow on Antarctica about 60 million years ago and ice sheets have intermittently covered portions of Greenland since 7 million years ago.

Atmospheric CO₂ has been relatively low for the past few million years. During this time, changes in Earth’s orbit have been the dominant driver of climate change and have caused about ten glacial maxima, otherwise known as “ice ages.” The evidence of past climates can be found in sea sediments, tree rings, and cave rocks. Recently, the impact of orbital changes has been amplified by the reflectivity of growing ice sheets and the absorption of CO₂ into the ocean.

During the Last Glacial Maximum (LGM), atmospheric CO₂ was about 185 ppm (parts per million), which is about 90 ppm less than pre-industrial levels and more than 200 ppm lower than today. Several factors affect how much CO₂ is absorbed by the ocean, including ocean temperature, biologic activity in the ocean, and ocean circulation (particularly the rate at which deep ocean water mixes back up to the surface). Paleoclimate scientists are currently working to better understand the relative importance of these different factors in explaining the lower CO2 during the LGM.

Current CO₂ levels of approximately 400 ppm are higher than at any time in the past million years and are likely similar to the CO₂ levels 3 million years ago, when there was considerably less ice on Greenland and West Antarctica.

To read more about CO₂ measurements from ice cores, check out this site, which includes data collected from the British Antarctic Survey.

Without ice on Greenland and Antarctica, sea level would be 70 meters higher. Past sea levels were even higher because of the warming of the ocean. The ocean expands when it warms—and the size of ocean basins has changed as the result of plate tectonics.

During the LGM, the additional ice on land stored enough water to lower sea level by 130 meters. Sea level rose rapidly between 18,000 to 8,000 years ago as this ice melted.

Contributions to modern sea level rise include the melting of mountain glaciers and ice sheets in Greenland and West Antarctica, as well as the thermal expansion of seawater as it warms. The IPCC estimates that global mean sea level rise is likely to rise if fossil fuel emissions continue to follow historical trends. Therefore, reducing fossil-fuel emissions in the near future is important for reducing sea level rise.

Read more about observations of modern climate change and projections for future climate change here.

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