Opinion Scientists monitoring water flow in streams at test sites across the USA have found, unexpectedly, that the global warming seen in the late 20th century had basically no effect on most of the ecosystems they studied.
The world in general is thought to have warmed up by approximately half a degree C from 1980 to the year 2000, and while the past decade has seen no further increase, most full-time professional climate scientists expect warming to resume in the near future. However there has been much disagreement as to just what effects this could have.
It has often been suggested that the 20th-century warming alone would be sufficient to start causing noteworthy damage to various important ecosystems such as the wet forests of the US Pacific Northwest, which might in turn result in higher levels of atmospheric carbon going forward as trees died and decomposed - and then in future failed to absorb the large amounts of CO2 they normally would. Such postulated positive-feedback mechanisms provide much of the basis for forecasts showing rapidly-climbing global temperatures in this century.
For this reason, the US government has been establishing long-term monitoring facilities across its territory for decades now, allowing accurate records to be collected showing exactly what ecosystem impacts have occurred. Results are now in on 35 important headwater basins feeding river systems across the States over the last 20 to 60 years: and they show that in 28 of these, no effects on water flow from warming could be found at all.
Even where a warming-driven effect could be identified in the record, it was small compared to other more important factors such as "municipal and agricultural water usage, forest management, wildfire, hurricanes, and natural climate cycles".
"When presented with warmer and drier conditions, trees in the Pacific Northwest appear to use less water and therefore the impact on streamflow is reduced,” explains geographer Julia Jones. “In other parts of the country, forest regrowth after past logging and hurricanes thus far has a more definitive signal in streamflow reduction than have warming temperatures.”
According to a statement issued by Jones' university highlighting the new research:
Jones said the important message in the research is that the impacts of climate change are not simple and straightforward.
The full paper (pdf) is published in the journal BioScience.
Previous data from the US long-term environment monitor stations has also shown that droughts and heatwaves aren't nearly so big a deal for ecosystems as had been thought. ®
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