How do scientists know that recent climate change is largely caused by human activities?

• Raju Bhattarai

Scientists 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 reveal that this increase is due to human activities. Other greenhouse gases 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 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. Our knowledge of how greenhouse gases trap heat underlies the predicted shifts in climate. Natural factors alone are insufficient to explain the recent observed changes in climate, according to both this fundamental knowledge of the physics of greenhouse gases and pattern-based fingerprint investigations. 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 20^th century and into the 21^st. Only when models include human influences on the composition of the atmosphere are the resulting temperature changes consistent with observed changes.