Frequently Asked Questions

Frequently Asked Questions - Frequently Asked Questions...

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Frequently Asked Questions Frequently Asked Question 6.2 Is the Current Climate Change Unusual Compared to Earlier Changes in Earth’s History? Climate has changed on all time scales throughout Earth’s histo- ry. Some aspects of the current climate change are not unusual, but others are. The concentration of CO2 in the atmosphere has reached a record high relative to more than the past half-million years, and has done so at an exceptionally fast rate. Current global tempera- tures are warmer than they have ever been during at least the past five centuries, probably even for more than a millennium. If warm- ing continues unabated, the resulting climate change within this century would be extremely unusual in geological terms. Another unusual aspect of recent climate change is its cause: past climate changes were natural in origin (see FAQ 6.1), whereas most of the warming of the past 50 years is attributable to human activities. When comparing the current climate change to earlier, natural ones, three distinctions must be made. First, it must be clear which variable is being compared: is it greenhouse gas concentration or temperature (or some other climate parameter), and is it their abso- lute value or their rate of change? Second, local changes must not be confused with global changes. Local climate changes are often much larger than global ones, since local factors (e.g., changes in oceanic or atmospheric circulation) can shift the delivery of heat or moisture from one place to another and local feedbacks operate (e.g., sea ice feedback). Large changes in global mean temperature, in contrast, require some global forcing (such as a change in green- house gas concentration or solar activity). Third, it is necessary to distinguish between time scales. Climate changes over millions of
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Frequently Asked Questions - Frequently Asked Questions...

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