That was the conclusion of researchers who analysed the results of 40 previous studies that examined the links between climate change and alterations in the ecology of the continental United States and Alaska.

Among the most striking examples were changes in the spring timetable of certain populations of plant and animal species, such as the earlier blooming of the forest phox and the butterfly weed in the upper Midwest.

By the 1980s and 1990s, the plants were blooming 15 and 18 days earlier than they had 60 years earlier, according to the report for the Pew Center on Global Climate Change.

Those events were part of an accelerated spring calendar that saw spring events occurring seven days sooner, on average, than they had six decades before -- a recalibration that coincided with a temperature increase of three degrees Celsius (five Fahrenheit).

Another study documented the changing hibernation patterns of yellow-bellied marmots living in the Rocky Mountains. By the 1990s, they were emerging from hibernation 23 days earlier than they had 15 years before -- again in line with rising temperatures.

The authors of the study also pointed to the extinction of certain populations of Edith's checkerspot butterfly in southern and low level elevations, in Mexico and Baja California, over time.

The extinctions were probably temperature-related insofar as the plants in those regions that host the butterfly's caterpillar precursor had started to dry up before the caterpillars could fully develop.

The effect has been to reconfigure the species' habitat, with populations concentrated northwards, in Canada, and upwards in elevation, in the Sierra Nevada mountain range, where temperatures are cooler.

In some cases the shifts do not appear to have had any negative impact, but in others, they have made survival tougher as the large scale movements bring new species into contact with each other.

As temperatures have warmed in parts of Canada, the red fox has shifted northwards and is now encroaching on the Arctic fox's range, threatening its survival, the study said.

"The conclusion from these studies is that global climate change has affected about half of all wild species," said Camille Parmesan, a biologist at the University of Texas at Austin.

Scientists estimate that greenhouse gases have increased the temperature in the 48 contiguous US states by one degree Fahrenheit (0.6 Celsius) on average in the past century. But some parts of Alaska have witnessed temperature changes of up to seven degrees Fahrenheit (3.9 Celsius).

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