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Carbon dioxide, or CO2, is a very common naturally occurring molecule that contains two oxygen atoms and one carbon atom. In everyday conditions on Earth, carbon dioxide is a common gas around us. It is colorless, odorless, is naturally present in the Earth's atmosphere, and is an important part of the Earth's carbon cycle. All humans and animals exhale carbon dioxide when they breathe, and plants absorb it during a process called photosynthesis to grow.
CO2 is called a greenhouse gas (GHG) because, as part of Earth's atmosphere, it traps energy from the sun and keeps the world at a habitable temperature. But increases in atmospheric CO2 associated with human activities can pose problems. For example, on the one hand, the burning of fossil fuels releases more CO2 into the atmosphere (along with other greenhouse gases) and, on the other hand, the destruction of forested areas makes trees absorb less CO2, etc. Both cases carry too much energy or heat being trapped in our atmosphere. This additional energy causes increased climatic instability, resulting in major changes in weather patterns.
Change in temperature and change in carbon dioxide
One of the most remarkable aspects of the paleoclimate record is the strong correspondence between temperature and the concentration of carbon dioxide in the atmosphere, observed during the glacial cycles of the last hundreds of thousands of years. When the concentration of carbon dioxide increases, the temperature increases. When the concentration of carbon dioxide drops, the temperature drops.
A small part of the correspondence is due to the relationship between temperature and the solubility of carbon dioxide at the ocean surface, but most of the correspondence is consistent with a feedback between carbon dioxide and climate. These changes are expected if the Earth is in radiative equilibrium and are consistent with the role of greenhouse gases in climate change.
While it may seem straightforward to determine cause and effect between carbon dioxide and the climate from which the change first occurs, or by some other means, determining cause and effect remains extremely difficult. Additionally, other changes are involved in the glacial climate, including altered vegetation, land surface characteristics, and the extent of the ice sheet.
Other paleoclimatic approaches help us understand the role of the oceans in past and future climate change. The ocean contains 60 times more carbon than the atmosphere, and as expected, the changes in carbon dioxide in the atmosphere paralleled the changes in carbon in the ocean over the past hundreds of thousands of years. While the ocean changes much more slowly than the atmosphere, the ocean played an essential role in past variations in carbon dioxide, and will play a role in the future for thousands of years.
Finally, paleoclimate data reveals that climate change is not just about temperature. As carbon dioxide has changed in the past, many other aspects of the climate have also changed. During the ice ages, the snow lines were lower, the continents were drier, and the tropical monsoons were weaker. Some of these changes may be independent; others closely coupled to the changing level of carbon dioxide. Understanding which of these changes may occur in the future, and how large those changes may be, remains a vigorous research topic. NOAA's Paleoclimatology Program helps scientists document changes that have occurred in the past as an approach to understanding future climate change.
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