Human Influences and Their Timescales

Key Warming and Cooling Influences
Key Warming and Cooling Influences
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While fossil fuel combustion is the largest source of greenhouse gas emissions that cause global warming, many other human activities also have local, regional, and global climate implications. Some of these activities have a warming influence, while others exert a cooling influence.

For example, black soot (from diesel exhaust, wood smoke, and power plants) absorbs incoming sunlight, thus contributing to warming. Soot falling on Arctic snow and ice accelerates snow and ice melt, contributing to further warming.

Coal-fired power plants inject very large quantities of heat-trapping carbon dioxide into the atmosphere, but they also emit reflective sulfate particles, which have a cooling influence. But there is a critical difference between sulfate particles and carbon dioxide: The cooling particles injected by volcanoes and power plants are cleansed from the atmosphere within a few weeks, months, or years. Carbon dioxide, by contrast, is very long-lived and thus steadily accumulates in the atmosphere over centuries and millenia.

Some of the carbon dioxide produced hundreds of years ago, at the beginning of the industrial revolution is still in the atmosphere today, and will be contributing to warming the planet for centuries to come. Some of the carbon dioxide produced by our parents is still in the air today, and ours will be too, when our grandchildren grow old. The very long-lived nature of carbon dioxide in the atmosphere is a major reason that climate change will continue to some extent, even after the world adopts policies to reduce greenhouse gas emissions.

However, choices made today will determine whether the world experiences a relatively small amount of additional climate change, or a catastrophic amount. A recent analysis suggests that all the power plants, factories, buildings, and cars that exist today would not produce enough carbon dioxide over their useful life to push climate past a dangerous level of warming. That is the good news: we are not yet committed to catastrophic warming. But the bad news is that we are still building more carbon-emitting equipment every day, and it is those new power plants, cars, and other devices that will push the climate into dangerous territory, unless we act quickly to ensure that new energy infrastructure will be primarily non-carbon-emitting. Rapidly developing and globally deploying those alternatives will require an extraordinary effort.