The Long, Hot Summer: Longer & Hotter

Stanford study predicts the point of no return for hotter summers

By Katrina Schwartz

Just as many Californians are puzzling over winter-like weather in June, climate scientists are saying hotter days are ahead for most of the West. According to a new Stanford study (available soon at this link), we may be in for permanently hotter summers sooner than expected. Of course, for climatologists, “sooner” is a relative term.

Photo: Craig Miller

Plenty of climate scientists have studied the relationship between climate change and extreme temperature shifts, but until now no one has tried to pinpoint a moment when summer temperatures will permanently shift into a new “heat regime”, in which the coolest summer temperatures will be hotter than the hottest summer temperatures of the previous regime. Findings by the Stanford team suggest that the shift will likely happen sooner and be more widespread than expected.

The research team led by Noah Diffenbaugh of the university’s Climate and Earth System Dynamics Group analyzed more than 50 climate model simulations and estimated a 50% likelihood that a permanent shift will happen in tropical parts of the globe in the next twenty years. In middle latitudes like Europe and North America that shift will likely happen in 40 to 50 years, the study suggests. The authors say that because temperatures don’t vary as widely near the Equator, it won’t take as much warming to bump those regions into a new “seasonal envelope”— a completely new summer temperature range.

The Stanford team applied the same climate models to historical data to see how well they could predict what actually happened between 1979 and 2008. They concluded that many areas of the globe are already experiencing these permanent heat shifts. In central Africa, the authors conclude, 40% of the land area has already experienced a permanent upward shift. The climate models were able to predict the same results, making the observable reality match the simulated prediction. This correlation gives Diffenbaugh confidence in his team’s predictions for the future.

The study has potentially dramatic effects on humans. Drastically warmer temperatures adversely affect human health and agriculture. Morbidity and mortality rates rise. The demand for energy increases while the ability to supply it decreases. Many crops important to the economy of the western United States like grapes, corn, soybeans, and cotton cannot handle extreme heat. While the study found that only the eastern and western parts of the U.S. would experience permanent summer temperature increases, Diffenbaugh was quick to point out to me that his team studied the most dramatic shift possible — a complete shift upward in temperature into a new seasonal range. He says that the effects on areas that don’t experience a permanent shift in the next 50 years — like the Midwest — could still be significant.

Diffenbaugh says he was intrigued by the wintertime comparisons in the study. He explained that the relative seasonal sameness of the tropics causes the bump up in temperature to happen quickly. In the mid-latitudes, however, the move into hotter regimes takes much longer because of overlays like arctic air movement that occur simultaneously to an overall warming trend.

Katrina Schwartz is former KQED News intern, now a freelance contributor to Climate Watch.