If you're in the Continental US at the moment, you're probably giving lots of thought to how the wind chill makes things feel notably worse than the frigid temperatures might indicate. And, if you're not, your local weather forecaster will probably be happy to inform you of what it really feels like outside, regardless of what the temperature says.
This isn't something meteorologists made up to make cold snaps feel worse; the wind really does enhance the chilling effect of cold air. And it works on the other side of the comfort zone as well, as humidity combines with high temperatures to enhance heat stress.
The rising temperatures driven by climate change are expected to change human comfort levels in a way that mostly balances out—uncomfortable heat in the tropics is expected to be offset by fewer cold problems at higher latitudes. But rising temperatures will also affect wind patterns and humidity levels, so there's a chance that things will feel different from simply "it's a bit warmer." To find out, a Canadian-Chinese research team used a set of historical records and climate models to figure out how humans are perceiving their changing climate.
The team combined measurements of air temperature near the Earth's surface (where people are generally located) with specific humidity at two meters and wind speed at 10 meters. These measurements could be combined to calculate the apparent temperature, or roughly how cold or hot a human feels. The apparent temperature was contrasted with the air temperature obtained in the same experiment. The temperature, wind, and humidity were taken from a set of historic weather records; future conditions were pulled from the output of a panel of seven climate models.
Overall, the combination of heat and humidity appears to be outpacing the drop in cold extremes. Even as the actual temperatures rose for much of the 20th century, they were outpaced by the perceived temperature, which rose 0.04 degrees Celsius faster each decade since 1950. Even if we get our emissions under control during this century, our perceived temperature will continue to get worse, accelerating to 0.06 degrees Celsius per decade over this century. If we continue with business-as-usual emissions, it will hit 0.17 degrees Celsius per decade.
The only scenario in which we don't feel more uncomfortable as the century goes on involves emissions beginning to drop over the next few decades. And, at least on land, there are no significant regions where the increase in wind speed adds a chill that offsets the warming of winter temperatures. In other words, even in the Arctic, we're likely to experience more days when summertime warmth feels warmer.
While the numbers may seem small, they add up. Over the course of this century, 0.06 degrees Celsius per decade would add up to it feeling like it's more than a full degree Fahrenheit warmer than it actually is—and that's on top of steadily rising actual temperatures. In addition, the problem isn't evenly distributed across the planet's surface. Mapping out the changes shows them heavily concentrated in the equatorial regions, which are already at risk of heating up to the point where sustained, outdoor human activities become dangerous. In these regions, humidity accounts for 30 to 40 percent of the change in perceived temperatures.
The equatorial regions, which will feel this the most, will be hit with a variety of other issues as the climate warms. They're already some of the poorest and most densely populated regions on the planet, and many of them are at risk from sea-level rise and problems maintaining agricultural productivity. The added unpleasantness of simply being outdoors would increase the factors that could drive waves of migration out of these regions.