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Why the Climate is Changing

Greenhouse Gas Emissions and the Greenhouse Effect

The Science of Climate Change

For more than a century, physicists have known that certain gases in the atmosphere absorb heat, making the planet's overall temperature higher than it would be if these gases were not present. This is called the "greenhouse effect".

Greenhouse gases (GHGs) in the atmosphere allow short-wave radiation (like sunlight) to pass through and warm the Earth, but prevent long-wave radiation (heat) from escaping back out to space. Heat trapped by this greenhouse effect raises the temperature of air, land, and water.

Greenhouse Gas Emissions

Globally, CO2 makes up about three-fourths of all human GHG emissions. The other one-fourth - methane, nitrogen oxides, ozone and the synthetic industrial GHGs - are gases that trap heat even more effectively than CO2.

Diagram showing greenhouse effect

To evaluate the full "carbon footprint" of human emissions, scientists use a measurement unit that takes into account the different warming characteristics of individual GHGs. The "carbon dioxide equivalent" (CO2e) expresses each GHG's warming impact in terms of the amount of CO2 that would create the same amount of warming.

These substances are released by human activities and accumulate in the atmosphere. CO2 and other long-lived GHGs can persist for hundreds, or even thousands, of years. Since industrialization began, CO2 has built up in the atmosphere from about 280 to more than 400 parts per million - and GHG emissions are continuing to increase. After a century and a half of human GHG emissions, the atmosphere holds more total GHGs than at any time in at least 800,000 years.

In only about a century and a half, two types of human activities have injected into our atmosphere vast stores of carbon that had remained sequestered under the Earth for millions of years:

  • Combustion of fossil fuels (coal, oil and natural gas) oxidizes carbon in long-buried ancient plants to form the greenhouse gas CO2. As our chief source of energy for generating electricity, heating buildings and operating vehicles, fossil fuels contribute most of the CO2 we emit. Some ancient carbon has been sequestered chemically, for example as calcium carbonate in limestone rock; these compounds convert to CO2 when the rock is heated to make cement.
  • Deforestation and land use change release the carbon stored more recently in trees and soils. Besides adding significant CO2 to the atmosphere's GHG load, deforestation diminishes the biosphere's present and future capacity to remove CO2 from the atmosphere.
Diagram of carbon cycle

Humans' massive infusion of CO2 has unbalanced Earth's carbon cycle by adding more carbon than natural processes can remove.

Global Warming

As the heat-trapping "blanket" of GHGs in the atmosphere grows denser, it retains more and more heat and the earth warms. Earth's weather is driven by heat energy. As trapped heat builds up in the land, oceans and lower atmosphere, the predictable weather patterns that we call "climate" are changing.

Science is able to project how the earth's climate will respond as GHGs accumulate, but how fast and exactly where specific changes will occur are still uncertain.

New York bases its climate policies on scientific studies, with regional downscaling of data to project the local consequences of ever-rising GHG concentrations.

Scientific Consensus

Many independent lines of scientific evidence underlie our understanding of "greenhouse" warming of the Earth, of how warming changes Earth's climate and of what role each of us plays in intensifying the greenhouse effect.

Studies conducted the world over have concluded:

  • Today, the Earth's atmosphere holds about 40 percent more heat-trapping greenhouse gases (GHGs) than it did only a century and a half ago; this increase is consistent with the amount of fossil fuel burned since the Industrial Revolution.
  • More GHGs mean a more intense greenhouse effect -- more heat trapped on Earth, less heat escaping into space.
  • GHGs now in the atmosphere are known to be generated by human activities, especially by deforestation and burning of fossil fuels (signaled by a slight but steady decrease in the atmospheric proportion of an isotope of carbon that is deficient in plant-based fuels).
  • Heat trapped by excess GHGs is warming the Earth -- raising sea levels by expanding sea water and melting land ice; energizing the climate system, which disrupts familiar climates and sets off extreme temperatures and precipitation.
  • The Earth is, on average, 1.5 degrees Fahrenheit warmer since reliable temperature measurements began in 1880. And warming is speeding up -- each of the last three decades was warmer than the previous decade; the ten warmest years on record all occurred since 1997.

More about Why the Climate is Changing: