How air circulates around our planet is being affected by climate change in ways that will pose real challenges for humanity. That’s according to two studies highlighted this week, one looking at convection – the movement of air vertically through the atmosphere – and one at Earth’s surface winds. The first brings good news on how often hurricanes will happen, but more worrying news on the combination of temperature and humidity humans must endure. The second underlines that in order to get the most out of wind power, turbines must be installed before temperatures rise too greatly.
Hurricanes and tropical ocean thunderstorms are aspects of the atmospheric convection that tends to happen when the sea surface passes a certain temperature limit. Writing in Nature Geoscience on Sunday, University of Hawaiʻi at Mānoa’s Nat Johnson and Shang-Ping Xie found that this threshold has been rising in parallel with the world’s average sea surface temperature. “The correspondence between the two time series is rather remarkable,” said UH Mānoa researcher Johnson. “The convective threshold and average sea surface temperatures are so closely linked because of their relation with temperatures in the atmosphere extending several miles above the surface.”
On one hand, this is good news because it means it is unlikely that the frequency of hurricanes and tropical storms will rise. But on the other it will pose problems for humanity as it means we will be exposed to more dangerous combinations of heat and humidity, a threat underlined by Steven Sherwood from the University of New South Wales, Australia, earlier this year on this blog.
“In general, it now takes more heat and moisture near the surface to induce the thunderstorms that cool the atmosphere below,” Johnson told Simple Climate. “If the sea surface temperature threshold did not rise, then we might expect longer hurricane seasons, an expansion of the hurricane development regions, and quite possibly substantially more hurricanes. Because we find that the sea surface temperature threshold rises with the tropical average sea surface temperature, we cannot reach such dire conclusions.” Johnson does emphasise, however, that nothing in his results contradicts the widely held belief that the most intense tropical cyclones will become more common with global warming.
Sherwood used a measure called the “wet-bulb temperature” that combines humidity and temperature. If the human body is exposed to conditions beyond 35ºC by that measure, it cannot regulate heat well enough and heat stress becomes extremely dangerous. “They discuss that in the current climate the wet-bulb temperature never – or almost never – exceeds 31ºC,” Johnson said. “The reason they cite for this limit relates to the threshold for convection. Whenever the wet-bulb temperature reaches this level, the combination of heat and moisture is almost always sufficient to induce thunderstorms, which cool the air below and bring the wet-bulb temperatures back down.”
“Convection serves as the “thermostat” that limits wet-bulb temperatures and therefore maximum heat stress in today’s climate to about 30ºC,” Sherwood himself echoed. “The upshot of the new study is that the thermostat setting is rising with global temperature.”
“Whether it would have been better for the thermostat not to increase, is hard to say” Sherwood told Simple Climate. “In that alternative world, you’d see rapid increases in severe storm activity with each degree of global warming, which would be terrible for anyone living in hurricane-prone areas. On the plus side, this mounting storminess would prevent peak heat stress from worsening. But I would never have reckoned that to be a realistic scenario, and it now looks even more unlikely based on this new study.”
Meanwhile, countries relying on wind energy to supply clean power are set to get less juice for their buck as global warming continues, Diandong Ren has highlighted this week. The University of Texas, Austin, researcher has found that as temperatures rise, electricity generated by wind turbines will fall. “The earlier we switch to clean energy, and thereby decrease the global climate warming trend, the more cost-effective will be the harnessing of wind energy,” he wrote in the latest issue of the Journal of Renewable and Sustainable Energy.
Winds at the level of the atmosphere where turbines are found are caused by differences in temperature across the planet. “The stronger the temperature contrast, the stronger the wind,” Ren said. As the climate changes and global temperatures rise, the temperature differences between the poles and the rest of the world falls. “The poles will tend to warm faster, since the comparatively small polar regions must balance extra heat from the rest of the globe,” Ren wrote. This effect is amplified as the bright white polar ice caps melt and become ocean, and absorb the sun’s warming energy instead of reflecting it back out into space. As the temperature contrast becomes weaker, so too do winds.
Measurements have already shown falling windspeeds in the US, cautiously linked to climate change. However Ren specifically studied how much wind energy would be available to generate power, based on the predictions generated by eight different climate models. He also focused on China, and used the specifications of General Electric’s 1.5 megawatt xle turbines as the basis of his calculations. He found that a 2-4ºC increase in temperatures outside the poles would result in a 4-12 percent decrease in wind speeds in certain northern locations. “All eight models indicate that accessible wind energy will degrade over the course of this century,” Ren wrote. Consequently, wind turbines deployed early enough to stop warming would generate more electricity than the same number deployed after warming has happened, offering a real economic motivation for earlier roll-outs.
Ren also suggests that the situation could turn out to be worse than his predictions suggest. “The climate models’ simulated reduction of poleward temperature gradient may underestimate reality,” Ren wrote. This is because the climate models have not incorporated recent findings that the greenhouse gas methane could be released from permafrost frozen soils and the icy Arctic oceans, amplifying warming still further. He also noted that the models do not completely simulate the effects of melting land-ice in regions like the Himalayas, which is especially relevant to China.