How can we identify global warming?
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Most Americans recognize climate change, but some are still unsure about its causes. Tens of thousands of scientists in more than a hundred nations have amassed an overwhelming amount of evidence pointing to a clear conclusion: Humans are the main cause of climate change. We're the ones who burn fossil fuels, produce livestock
and clear trees, increasing the amount of heat-trapping gases in the atmosphere. No one questions the link between smoking and cancer, because the science was settled in the 1960s after more than 50 years of research. We can think of the state of human activities and climate change as no different than smoking and cancer. In fact, we are as confident that humans cause climate change as we are that smoking causes cancer.
So what's the evidence?The research falls into nine independently studied, but physically related, lines of evidence:
Act when it matters mostEvery day more than 60 people sign up for news and alerts, to find out when their support helps most. Will you join them? (Read our privacy statement.) Donate to support this workScientists know that recent climate change is largely caused by human activities from an understanding of basic physics, comparing observations with models, and fingerprinting the detailed patterns of climate change caused by different human and natural influences. Since the mid-1800s, scientists have known that CO2 is one of the main greenhouse gases of importance to Earth’s energy balance. Direct measurements of CO2 in the atmosphere and in air trapped in ice show that atmospheric CO2 increased by more than 40% from 1800 to 2019. Measurements of different forms of carbon (isotopes, see Question 3) reveal that this increase is due to human activities. Other greenhouse gases (notably methane and nitrous oxide) are also increasing as a consequence of human activities. The observed global surface temperature rise since 1900 is consistent with detailed calculations of the impacts of the observed increase in atmospheric greenhouse gases (and other human-induced changes) on Earth’s energy balance. Different influences on climate have different signatures in climate records. These unique fingerprints are easier to see by probing beyond a single number (such as the average temperature of Earth’s surface), and by looking instead at the geographical and seasonal patterns of climate change. The observed patterns of surface warming, temperature changes through the atmosphere, increases in ocean heat content, increases in atmospheric moisture, sea level rise, and increased melting of land and sea ice also match the patterns scientists expect to see due to human activities (see Question 5). The expected changes in climate are based on our understanding of how greenhouse gases trap heat. Both this fundamental understanding of the physics of greenhouse gases and pattern-based fingerprint studies show that natural causes alone are inadequate to explain the recent observed changes in climate. Natural causes include variations in the Sun’s output and in Earth’s orbit around the Sun, volcanic eruptions, and internal fluctuations in the climate system (such as El Niño and La Niña). Calculations using climate models have been used to simulate what would have happened to global temperatures if only natural factors were influencing the climate system. These simulations yield little surface warming, or even a slight cooling, over the 20th century and into the 21st. Only when models include human influences on the composition of the atmosphere are the resulting temperature changes consistent with observed changes. Page last updated: March 2020 Find out about the Royal Society's latest work on energy, environment and climate Was this page useful? Thank you for your feedback Thank you for your feedback. Please help us improve this page by taking our short survey. Share: |
