Many lines of evidence demonstrate that human activities, especially emissions of greenhouse gases from fossil fuel combustion, deforestation, and land-use change, are primarily responsible for the climate changes observed in the industrial era, especially over the last six decades. Observed warming over the period 1951–2010 was 1.2°F (0.65°C), and formal detection and attribution studies conclude that the likely range of the human contribution to the global average temperature increase over the period 1951–2010 is 1.1°F to 1.4°F (0.6°C to 0.8°C;15 see Knutson et al. 201716 for more on detection and attribution).
Human activities affect Earth’s climate by altering factors that control the amount of energy from the sun that enters and leaves the atmosphere. These factors, known as radiative forcings, include changes in greenhouse gases, small airborne soot and dust particles known as aerosols, and the reflectivity (or albedo) of Earth’s surface through land-use and land-cover changes (see Ch. 5: Land Changes).17,18 Increasing greenhouse gas levels in the atmosphere due to emissions from human activities are the largest of these radiative forcings. By absorbing the heat emitted by Earth and reradiating it equally in all directions, greenhouse gases increase the amount of heat retained inside the climate system, warming the planet. Aerosols produced by burning fossil fuels and by other human activities affect climate both directly, by scattering and absorbing sunlight, as well as indirectly, through their impact on cloud formation and cloud properties. Over the industrial era, the net effect of the combined direct and indirect effects of aerosols has been to cool the planet, partially offsetting greenhouse gas warming at the global scale.17,18
Over the last century, changes in solar output, volcanic emissions, and natural variability have only contributed marginally to the observed changes in climate (Figure 2.1).15,17 No natural cycles are found in the observational record that can explain the observed increases in the heat content of the atmosphere, the ocean, or the cryosphere since the industrial era.11,19,20,21 Greenhouse gas emissions from human activities are the only factors that can account for the observed warming over the last century; there are no credible alternative human or natural explanations supported by the observational evidence.10,22
