What is the overall direction of global air currents at the equator?

Air rises at the equatorost heat arrives in the tropics, but it can be re-radiated from the top of the atmosphere anywhere on Earth. Spreading it out allows a larger area of the atmosphere to participate in its return. This yields a more efficient disposal of heat, and that is what global atmospheric flows try to achieve. Warm air is transported towards the poles, cooled air returns equatorward. Currently Earth experiences a growth in the "greenhouse effect"--the effect of gases like CO2 (carbon dioxide), methane, water vapor and ozone, which impedes the flow of heat from the ground where it is first adsorbed, to the top of the atmosphere where it is radiated back to space. Global flows, described below, help drive heated air from the equator towards the poles, spreading the area of heat return. When the transport of heat from the ground to the top layers is impeded by the "greenhouse effect," such flows may be driven to spread the area participating in heat return, making them extend further poleward. The result of such spreading could be the recently observed melting of polar sea ice (which threatens polar bears) and of glaciers in Greenland and in Antarctica.

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The answer is trial 3.