As humans emit greenhouse gases into the environment, it causes the Earth to warm, we already know that. What is less certain is how it will cause changes to the weather we experience in our lives. In the past few years, research looking into the connection between a warming planet and more extreme weather has found more conclusive connections.

I have covered extreme weather quite a bit recently, because the science is so compelling and new. But a new paper published in the Proceedings of the National Academy of Sciences by doctoral student Omid Mazdiyasni and his advisor Amir AghaKouchak takes a fresh look at this topic.



Instead of just looking at heat waves or just looking at precipitation, they looked for concurrent events. Droughts can be caused by reduced precipitation. Hot weather speeds evaporation and damages the environment. But droughts and high temperatures can happen at the same time. These concurrent-event droughts are particularly harmful, they can set in fast and severely.

Lead author, Omid Mazdiyasni

The authors present results of various heat wave severity (85%, 90%, and 95% events) and for various durations (3 day, 5 day, and 7 day events). Focusing on 1960-2010, they found that the concurrence of all combinations of drought, heat wave intensity, and heat wave durations “have increased substantially in the south, southeast, and parts of the western USA.”



In other past of the country, these trends have decreased (I have written about changing precipitation in the USA here and I have published on this topic here), in part because a warmer atmosphere contains more water vapor.

What is also interesting is that the authors find a greater increase in the most extreme events. The new study clears up past research which has been mixed in this area. The authors more advanced statistical technique is better suited to finding trends and patterns in the climate record.

Research director, Amir AghaKouchak Photograph: Steve Zylius/UC Irvine

I asked Omid Mazdiyasni about what makes his study unique. He told me,



To determine whether there has been a significant change in concurrent droughts and heatwaves, we used a statistical model new to hydrology. Our results show that although there is no statistical change in droughts individually, there is a significant change in concurrent droughts and heatwaves. The effects of this concurrent climatic event can greatly increase the damage on human health, air quality, agriculture, society, and the environment.

Next, I asked Amir AghaKouchak about the significance of this work and how it fits into the scientific literature on the topic. He responded,



In the past decades, the science community has focused mainly on individual climate extremes such as droughts, floods and heatwaves. We know very little about causes and impacts of concurrent and compound extremes. This study was a step forward toward this direction. But there is a lot to be done!



This is another study showing that the changing climate is causing real changes to the weather we experience. This paper doesn’t even include the recent droughts and heat waves of 2011, 2012, and the current drought in the western USA including California. These extremes present financial and social costs that will increase as we continue to warm the planet.