FIGURE 2. Changes in the westerlies and atmospheric structure in response to different CO₂ concentrations.

Bands of westerly winds in the Northern Hemisphere and Southern Hemisphere (shown schematically by the isotachs) separate the warm air (red shades) in the tropics from the cold air (blue shades) over the poles. a, Atmospheric structure today. Over recent decades, higher CO₂ concentrations have made the warm air warmer and the surrounding envelope of cold air cooler, especially near the top of the troposphere (curved red line). The thermal contrast across the zones of strong westerlies in the Northern and Southern Hemispheres is therefore greater, and the westerlies have become stronger and have shifted polewards in response. b, Proposed atmospheric structure at the LGM. With less CO₂ in the atmosphere, the thermal contrast in the middle of the atmosphere was probably decreased (indicated by paler shades), and the westerlies aloft should therefore have been relatively weak. The strongest westerlies were also significantly north of the ACC, where they would have had much less impact on the ocean.