Is climate change caused by human activity?

The latest Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) states that warming of the climate system is unequivocal, and that it is extremely likely (>95% confidence) that human influence has been the primary cause of warming. Scientists know that 20th and 21st century climate change is largely caused by human activities from:

  1. Basic physics: since the mid-1800s, scientists have known that CO2 and other “greenhouse gases” influence Earth’s energy balance. Observations of warming during the 20th and 21st centuries match theoretical calculations based on the rise in atmospheric CO2.
  2. Comparing observations with models: climate model simulations which only include natural influences such as the sun and volcanic eruptions don’t match the observed rise in global temperature.
  3. Fingerprinting: different influences on climate are known to leave different signatures or ‘fingerprints’ in historic climate records. Human influences can be identified in the spatial patterns of climate change observed over the Earth’s surface and through the atmosphere and ocean.

What about natural sources of CO2?

Natural systems, such as plants and animals on the land and in the oceans, produce a large amount of carbon, but they absorb it too. This cycle has been delicately balanced for thousands of years, but human influence has offset that balance to send the levels of CO2 rising. Natural sinks such as the ocean and vegetation are absorbing about half the greenhouse gases emitted by human activities, so they have actually reduced the amount that would otherwise have ended up in the atmosphere.

What about volcanoes?

Erupting volcanoes can have a significant effect on our climate. They emit large amounts of 'aerosols' (suspended dust particles, which have a cooling effect as they reflect the sun's energy back into space) as well as CO2. However, volcanic emissions of CO2 are well below the levels emitted by human activities over the past century.

What about changes in solar energy?

Changes in the amount of solar energy the earth receives from the sun can impact our global climate. Changes in solar output may have contributed to some of the warming observed in the early 20 century, but these changes explain less than 10% of the increase in global average surface temperature that has been observed since the late 19 century.

Isn't it just a natural cycle?

There is natural variability in Earth's climate, but the rate and persistence of current climate change cannot be explained by any natural climate cycles. Natural phenomena such as El Niño and La Niña, and volcanic eruptions cause changes in global temperature on relatively short timescales (months to years), which can lead to periods with little or no warming, both globally and regionally, and other periods with very rapid warming. The long-term trend, however, is clear that human activities have been the dominant cause of the observed rise in global temperature since the mid-20 century.


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