What causes climate change?

Climate is commonly thought of as the expected weather conditions at a given location over time. Climate can be measured as many geographic scales – for example, villages, districts, regions, countries, or the entire globe – by such statistics as average temperatures, average number of rainy days, and the frequency of droughts. Climate change refers to changes in these statistics over years, decades, or even centuries.

Enormous progress has been made in increasing our understanding of climate change and its causes, and a clearer picture of current and future impacts is emerging. Research is also shedding light on actions that might be taken to limit the magnitude of climate change and adapt to its impacts.

97% of climate scientists agree:

  • Climate change is happening now.
  • It is being driven primarily by human activity.
  • We can do something to reduce its impacts and progression.

What’s the difference between weather, climate, climate variability and climate change?

Weather is the temperature, humidity, precipitation, cloudiness and wind that we experience in the atmosphere at a given time in a specific location.

Climate is the average weather over a long time period (30 – 50 years) in a region.

Climate variability refers to natural variation in climate that occurs over months to decades. El Niño, which changes temperature, rain and wind patterns in many regions over about 2 – 7 years, is a good example of natural climate variability, also called natural variability.

Climate change is “a systematic change in the long-term state of the atmosphere over multiple decades or longer.” Scientists use statistical tests to determine the probability that changes in the climate are within the range of natural variability — similar to the statistical tests used in clinical trials to determine whether a positive response to treatment is likely to have occurred by chance. For example, there is a less than 1% chance that the warming of the atmosphere since 1950 could be the result of natural climate variability.

What causes climate change?
At its most basic, climate change is caused by a change in the earth’s energy balance — how much of the energy from the sun that enters the earth (and its atmosphere) is released back into space. The earth is gaining energy as we reduce the amount of solar energy that is reflected out to space — just like people gain weight if there is an imbalance between calories in and calories out. Since the Industrial Revolution started over 200 years ago, human activities have added very large quantities of greenhouse gases (GHG) into Earth’s atmosphere. These GHG act like a greenhouse (or a blanket or car windshield) to trap the sun’s energy and heat, rather than letting it reflect back into space. When the concentration of GHG is too high, too much heat is trapped, and the earth’s temperature rises outside the range of natural variability. There are many GHG, each with a different ability to trap heat (known as its “global warming potential”) and a different half-life in the atmosphere. GHG are sometimes called “climate active pollutants” because most have additional effects, most notably on human health.

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