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In Depth

Science
| 03/02/15

Temperature and precipitation: climate change’s dynamic duo

TCE

Temperature, not precipitation, is the primary driver of California’s droughts over the past two decades, including the current one now entering its fourth year, according to a new study published Monday. The Proceedings of the National Academy of Sciences (PNAS) paper examines climatic trends in California (CA) related to these two factors. While temperature and precipitation are relatively independent of each other, together (along with other factors) they can have a major impact on a region. The study found that human-caused climate change has yet to substantially reduce CA’s annual average precipitation but average temperatures across the state have gone up. That means more net evaporation, even with ‘normal’ rainfall. Contrary to popular belief, drought doesn’t just mean a lack of rain: it also means a lack of moisture in the soil. Thus as temperatures rise, even if precipitation remains the same, California can expect more intense and prolonged droughts in the future.

The interplay between temperature and precipitation is complex, but understanding the synergistic effects of these two critical variables is key to projecting and interpreting climate change. For example, an area could become hotter, but whether becoming hotter and drier or hotter and wetter will lead to vastly different effects on both people and ecosystems. Some regions today have very special ecosystems, where small changes in climate could impose not just subtle adjustments but a complete regime change. A recent study calculates the effects of climate change on one such unique ecosystem; North American temperate rainforests that run from Alaska all the way down to California. What makes this ecosystem unique is its delicate temperature and precipitation regime, which keeps it damp enough for the rainforest ecosystem to thrive. In Alaska, forest cover may spread as warmer temperatures allow southern species to shift northward; in BC, rainforests are also expected to creep upward to higher elevations, whereas in California iconic species such as the California redwood are severely threatened., The paper points out that few of the jurisdictions involved unfortunately have protected land areas large enough to serve as refuges to protect these migrating ecosystems. Addressing this shortage, for example by expanding the area of parks, is one concrete step that governments can take as part of a comprehensive climate-adaptation policy.

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