by Katie Dagon
figures by Utsarga Adhikary
Heat waves and droughts are staples of summer weather in America. This summer has proven to be no different! Much of the central and eastern U.S. endured a strong heat wave in mid-July. While the “heat dome” has subsided, parts of the Northeast are still under a severe drought (see Figure 3). What makes a particular period of hot or dry conditions extreme is different depending on where you live and the normal climate and weather conditions of that region. Climate change also plays a role in the weather we experience during the summer. Science has shown us that heat waves are predicted to intensify in the future due to climate change, in terms of their magnitude, frequency and duration. Drought and arid conditions are also predicted to increase under global warming. So is this extreme summer weather all due to climate change, or is some of it normal?
Before we get to the role of climate change in causing heat waves and drought, you should get an idea of what defines them. A heat wave refers to a period of time with above average temperatures and sometimes with above average humidity (excess moisture in the air). A common definition used in New England is three or more days where the air temperature exceeds 90°F, but this might be different than how it actually feels outside depending on humidity. That’s why the National Weather Service calculates a heat index, combining temperature and humidity. Their chart (Figure 1) includes health-related thresholds that help determine when to issue heat advisories and warnings.
Drought entails a deficiency in the amount of surface moisture, either through precipitation like rain or snow, or water stored in soils, rivers, or streams. Droughts are often categorized as meteorological (lacking rain), agricultural (lacking soil moisture), hydrological (lacking surface water), or socioeconomic (affecting humans). While these sub-categories aren’t mutually exclusive, they help to explain the impacts of the drought, and also provide some intuition into the cause. For example, an agricultural drought driven by a lack of soil moisture can be caused not only by a decrease in rainfall, but also an excess of evaporation from the land surface that depletes the water stored in the soil.
Can We Blame it All on Climate Change?
Recently the National Academies came out with a report to help answer this question. The report studied how extreme events, such as heat waves and droughts, can be attributed to climate change. In the past it has been hard for scientists to connect one specific event to the impacts of climate change. This report recognizes that for certain events, scientists can estimate the impact of human-caused climate change. This is based on three things: 1) how well we can simulate these events using climate models, 2) the observational record we have for these types of events, and 3) our physical understanding of how climate change will affect extremes. In the report, each type of extreme event was assessed for the three categories to calculate an overall level of confidence. Extreme heat and drought rank on the higher end, in terms of confidence in attributing these types of events to climate change (Figure 2). This is in contrast to things like severe thunderstorms and hurricanes, which are difficult to attribute to climate change because they are hard to simulate and tend to be more rare.
Recent Extreme Weather
We’ve already seen some examples of extreme weather this summer. During the heat wave last month, the heat index in many places reached over 100°F. While it’s not unusual for extreme heat to occur this time of year, climate change is increasing the seriousness of these events. As we saw with the recent heat wave, health-related impacts from extreme heat can be severe.
The drought in the Northeast persists, but has varied in intensity over the course of the summer. The strength of a drought is quantified by the U.S. Drought Monitor, which calculates several drought metrics. These metrics are based on the amount of local rainfall, as well as measurements of how much water is stored in the soil (soil moisture) and how much water is running through streams (called streamflow). Precipitation in the Northeast has been below average since February 2015, with climate conditions slowly building towards an extended dry period. Because this drought has lasted only a few months, we can’t yet fully assess the role of climate change in causing it, though it has likely played a role.
While the drought in New England has so far lasted only one season, other parts of the U.S. have been under drought conditions for much longer. The California drought began more than four years ago and in some areas is currently categorized as an “Exceptional Drought”, the highest ranking used by the U.S. Drought Monitor (Figure 3). This drought has already had enormous impacts on agriculture, wildlife, and water resources. In 2015, burning from wildfires affected three times the normal area. The U.S. Drought Monitor estimates that there are over 33 million people living in California drought areas, or roughly 90% of the state population. Due to the severity of this drought, many scientists have begun to examine the role of climate change. Studies have concluded that human-induced warming has helped to intensify drought conditions and make future drought more likely.
What about El Niño?
Some of you may remember reading about a so-called “Monster” or “Godzilla” El Niño that was predicted to bring much-needed rain to the western U.S. this past winter. Despite hopeful predictions and the anticipation of increased precipitation, the California drought persists today. This left many wondering: what happened? First of all, the predicted El Niño rains were never going to be enough to make up the water deficit. A lot of this has to do with how long California has been in drought: it’s going to take more than a single rainy season or one strong El Niño to resolve these extreme drought conditions. Another aspect is the unpredictability of El Niño impacts. While the 2015-2016 event was a very strong one, it’s never guaranteed where and how its most intense impacts will be felt. We still have a lot to learn about how El Niño is affected by climate change, and how it might evolve in the future.
A Coupled System
This summer, drought has occurred in both the Northeast and the West, regions where higher-than-normal summer temperatures were predicted (Figure 4). It may not be surprising that heat and drought go together, but what comes first: the heat or the drought? It actually works both ways. A strong heat wave with low humidity can inhibit precipitation, leading to drought. On the other hand, drought and especially low soil moisture can encourage heat waves by reducing the cooling effect of evaporation. Climate change can factor into the timing of heat versus dryness, as can local conditions like vegetation and topography.
What does this mean for the future of summer weather? If you live in the Northeast, those hot and humid days of peak summer aren’t going away any time soon. If you live in the West, dry conditions will persist until enough rain and snow can fall to make up for the years-long drought. But we’re getting better at understanding how extreme events are shaped by climate. For heat waves and drought, there is little doubt that these events are strongly related to the effects of climate change and we’ll continue to see more of them in the future.
Katie Dagon is a PhD Candidate in the Department of Earth and Planetary Sciences at Harvard.
For more information
- National Academies Attribution of Extreme Weather Events in the Context of Climate Change: http://www.nap.edu/catalog/21852/attribution-of-extreme-weather-events-in-the-context-of-climate-change
- National Weather Service Heat Safety Resources: http://www.nws.noaa.gov/om/heat/index.shtml
- S. Drought Monitor: http://droughtmonitor.unl.edu/
- NOAA video on Seasonal Outlooks: https://youtu.be/trUE7Vtfy5g