Irina Mahlstein, from the Swiss Federal Institute of Technology, in Zurich, and her colleagues used 23 atmosphere–ocean general circulation models to assess which locations will show evidence of anthropogenic global warming earliest. For each grid cell the team calculated the average climate variability going back to 1900. They then looked at forward projections for climate up to the year 2100 and identified the locations where climate change would exceed normal climate variability first.

They found that the earliest emergence of warming will occur in the summer season in low-latitude countries – between 25°N and 25°S. Their findings are published in Environmental Research Letters.

So why do low-latitude countries act as the canary in the coal mine? To detect global warming the local climate change needs to be at least two standard deviations from the mean base-period temperatures. "Regions with low variability have a rather narrow distribution and a smaller amount of warming is needed, which then also implies a smaller global warming," Mahlstein told environmentalresearchweb. "Regions with a high variability have wider distributions and therefore a larger warming is needed, and also a greater global warming."

The data shows that countries with the lowest inter-annual variability are situated in the region between 25° north and south. In particular low-latitude summers tend to be less variable than winters, making summer the most likely time for climate-change signals to emerge.

Already this signal has emerged in the Caribbean, and Mahlstein and colleagues' model suggests that many other tropical countries will follow suit over the next two decades. Unfortunately the impact of even a small degree of warming on these countries is likely to be more severe than for their high-latitude counterparts.

"It is already shown by other studies that for example ectotherms – animals not capable of regulating their body temperature – are affected, or that crop yields in these countries might be negatively affected," explains Mahlstein. "In addition, the low latitudes are mostly developing countries, which often depend strongly on agriculture, and therefore on the local climate. At the same time, those countries often have limited capacities in terms of technology and money to adapt to the changes or protect themselves."

The model also showed that anthropogenic global warming will become detectable in most countries worldwide when the mean global warming is 1 °C – far less than the total warming projected from any economically feasible emissions scenario. In the meantime it is regions like Japan, Indonesia, equatorial Africa and Central America that are likely to provide conclusive evidence that anthropogenic warming is well and truly under way.