Natural global warming facts
Massive volcanic eruptions can impact radiative forcing, the balance of radiation reaching the Earth and broadcast back into space (see References 1).
Over the past two centuries, human activity has increased the concentration of greenhouse gases (GHG) like carbon dioxide in Earth's atmosphere, and scientists believe these changes are causing a rise in global surface temperatures (see references 3). Human activities are not the only ones that can impact climate, however, and throughout Earth's history natural activities have also caused climate fluctuations (see references 3).
The sun's output shows small variations over the course of an 11-year cycle, so the average amount of solar radiation that reaches Earth's outer atmosphere, the TSI, varies by about 1.4 watts per square meter over this time frame (see references 1) (see references 4). These cyclic changes correlate with the number and frequency of "sunspots, " the so-called solar cycle (see references 3). Current evidence suggests the TSI has increased by about .12 watts per square meter since 1750, although there are significant uncertainties in these data (see references 3). TSI is presently about 1361 watts per square meter by way of comparison (see references 5).
Earth's position with respect to the sun also varies slightly over a longer time frame as a result of cyclic, predictable changes in Earth's orbit (see references 3). These changes are called Milankovitch cycles and scientists believe these are the culprit in Earth's past ice ages (see references 3). While these are very important for long-term fluctuations in climate, they have very little impact on shorter timescales because they take place so slowly (see references 3).
Water vapor is the most abundant GHG in Earth's atmosphere, although changes in its concentration are typically a result of temperature changes (see references 2). Consequently, water vapor can act as part of feedback loops, where an increase in temperature triggers an increase in water evaporation, which triggers an increase in temperature and so forth (see references 2). As the water vapor concentration increases, however, the water vapor also condenses into clouds, which reflect solar radiation and have an opposite effect (see references 2). Consequently, the role of these feedback loops and their importance is still poorly understood (see references 2).
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