Climate Basics

The Earth's climate is the result of three main factors: the incoming solar radiation (solar irradiance), which determines the energy input to the Earth system; the albedo of the Earth's surface, which determines the radiation reflected at the Earth's surface; and the amount of so-called greenhouse gases in the atmosphere, which determines how much of the long-wavelength radiation is captured by the atmosphere. If any of these three factors changes, climate will change. However, the spatial and temporal patterns of climate change can be complex and difficult to attribute to specific changes in any of the three forcing factors. The amount of heat stored or released in response to changes in the three forcing factors depends on the heat capacity of the main components, including the atmosphere, ocean, cryosphere, and solid land surface. Changes in the overall heat stored in the different components of the climate system have an impact on the dynamics within the climate system, in particular, the ocean and atmospheric circulation and the flows in the associated water cycle.

Climate can change on a wide range of spatial and temporal scales, and the changes in climate variables such as surface air temperature, air pressure, wind fields, air and soil moisture, precipitation, evapotranspiration, etc., can differ along these scales. For example, the changes in five-year averaged air temperature between 1900 and 2010 vary spatially between -2^oC and +4^oC, with the increases in the Arctic on average being much larger than in other geographical regions. Over time, global mean air temperature has varied over a total range of about 5^oC over the last 800,000 years (e.g., Hansen et al., 2008), but changes on century time scales have been very small (normally much less than 0.5^o C).

Solar irradiance exhibits small fluctuations over time. Most remarkable for human time scales is the solar sunspot cycle of approximately 11 years. The corresponding climate variations are minuscule, and the processes that cause the variations are still being researched (Rind et al., 2008).

The Earth's albedo, i.e., the ratio of reflected to incoming radiation, depends on the Earth's surface properties. Ice and snow-covered areas reflect most of the solar radiations, while open water and land areas reflect much less. Urban areas also have a lower albedo than, for example, grasslands and forests.

Without an atmosphere, the mean global temperature on the Earth's surface would be close to -18^oC, and this temperature would only depend on the incoming solar radiation. With the atmosphere present, the global mean surface temperature is today close to 16^oC, and this much higher temperature is mainly the result of the greenhouse gases in the atmosphere. The greenhouse gases most abundant in the atmosphere include H₂O, CO₂, CH₄, and N₂O, while other gases such as flourinated gases have high greenhouse potential but are less abundant in the atmosphere. If the atmospheric concentrations of greenhouse gases increase, then the surface air temperature also increases (Arrhenius, 1896).

However, due to the presence of the ocean, the full effect of a change in greenhouse gases can take a long time to develop. Water has a specific heat capacity more than 3000 times that of air, and the slow heating of the ocean thus delays the atmosphere in reaching the equilibrium temperature consistent with a given level of atmospheric greenhouse gases. Thus, the climate system reacts to changes in greenhouse gases not like a greenhouse but rather like a pool house, in which the swimming pool provides a large thermal mass that delays changes in the air temperature.

The warming ocean also contributes to changes in the climate dynamics. A warmer ocean evaporates more water, and the amount of water vapor that can be stored in air increases strongly with the air temperature. For example, at 40^oC, air can hold almost 5.5 times the water it can hold at 10^oC. Thus, warmer air can store much more water before condensation and precipitation happen, which results in longer intervals without precipitation followed by more extreme precipitation events. The water vapor represents latent heat, that is, energy that becomes available again when the water vapor condensates. This energy is available to drive storms. The warmer the air, the more latent heat it can hold and make available for storms once condensation is triggered.

Thus climate change is the result of changes in flows: flow of solar energy into the atmosphere-oceans system and storage of heat mainly in the ocean; the flow of water in the ocean currents that distributes energy throughout the ocean; the flow of energy in form of latent heat carried by water vapor from the oceans to the atmosphere; the transformation of latent heat into kinetic energy when the water vapor condensates; the flow of air in storms that can bring devastation to human and non-human communities, and the flow of water in the global water cycle that can cause floods and droughts.

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