The contributions to radiative forcing from some of the factors influenced by hu- man activities are shown in Figure 2. The values reflect the total forcing relative to the start of the industrial era (about 1750). The forcings for all greenhouse gas increases, which are the best understood of those due to human activities, are positive because each gas absorbs outgoing infrared radiation in the atmosphere. Among the greenhouse gases, CO2 increases have caused the largest forcing over this period. Tropospheric ozone increas- es have also contributed to warming, while stratospheric ozone decreases have contrib- uted to cooling. Aerosol particles influence radiative forc- ing directly through reflection and absorption of solar and infrared radiation in the atmo- sphere. Some aerosols cause a positive forcing while others cause a negative forcing. The di- rect radiative forcing summed over all aerosol types is negative. Aerosols also cause a nega- tive radiative forcing indirectly through the changes they cause in cloud properties. Human activities since the industrial era have altered the nature of land cover over the globe, principally through changes in croplands, pastures and forests. They have also modified the reflec- tive properties of ice and snow. Overall, it is likely that more solar radiation is now being reflected from Earth’s surface as a result of human activities. This change results in a negative forcing. Aircraft produce persistent linear trails of condensation (‘con- trails’) in regions that have suitably low temperatures and high humidity. Contrails are a form of cirrus cloud that reflect solar ra- diation and absorb infrared radiation. Linear contrails from global aircraft operations have increased Earth’s cloudiness and are esti- mated to cause a small positive radiative forcing