New Paper: What make heatwaves different and how do their impacts on ecosystems vary?

With global warming, heatwaves are becoming more frequent, intense, and prolonged, impacting ecosystems, food production, infrastructure, and human health. In our recent study published in Communications Earth & Environment led by Yinglin Tian, we show that heatwaves do not all operate in the same way. They arise from diverse processes that are reflected in characteristic differences in surface energy balance partitioning, each with distinct impacts on ecosystems and different trends under global warming.

Temperatures over land primarily reflect the energy balance between the heating and cooling fluxes at the surface. The surface is heated by absorption of solar radiation and the downward flux of radiation emitted by the atmosphere — the atmospheric greenhouse effect — while it is cooled by emitting radiation, evaporating water, and transferring heat into the atmosphere (that is, by turbulent fluxes). As a result, one can understand temperature variations by the associated changes in the terms of the surface energy budget. We have already applied this to study seasonal and climatological variations in temperatures (Ghausi et al., 2023) and extremely warm days and nights over the Tibetan Plateau (Tian et al., 2023).

In our new study, Yinglin Tian (now at PIK) and coworkers extend this framework and apply it to heatwaves. By clustering similar changes of different terms in the surface energy budget, we identified four main types of heatwaves: sunny–humid, sunny–dry, advective, and adiabatic. Each type has unique origins, patterns, and impacts on both ecosystems and human health.

Types of Heatwaves

This study classifies heatwaves based on underlying mechanisms, leading to four distinct types:

1. Sunny-Humid (38% of heatwave-days): They are driven by high solar radiation with a moderating effect from evaporation, which helps prevent temperatures from soaring higher. These heatwaves occur in high latitudes and tropical regions, including Canada, Europe, and Southeast Asia.
2. Sunny-Dry (26% of heatwave-days): They are characterized by clear skies, high solar radiation, and limited moisture. They are common in subtropical areas like the southern U.S., Brazil, and India.
3. Advective (18% of heatwave-days): Advective heatwaves are caused by hot air being transported horizontally from other regions. Found in cold and dry areas like Greenland and the Sahara, these heatwaves result from atmospheric circulation patterns and are associated with higher thermal stress on humans.
4. Adiabatic (18% of heatwave-days): Caused by air subsiding from higher altitudes and warming up. This type is triggered by increased downward radiation, which drives temperatures higher without additional solar input. They are primarily seen in mountainous and arid regions.

Distinct Impacts of Each Heatwave Type

The study found that each type of heatwave influences ecosystems and human health in different ways:

• Sunny-Dry Heatwaves: These have the most significant impact on ecosystems, particularly in regions with agriculture or dense vegetation. The lack of moisture during these events leads to reduced soil moisture and plant productivity, decreasing the net carbon uptake, which is a measure of how much carbon plants can sequester. In fact, sunny-dry heatwaves can decrease ecosystem carbon uptake by as much as 0.09 grams per square meter per day in harvested areas.
• Advective Heatwaves: While sunny-dry heatwaves strain plant productivity, advective heatwaves cause the greatest thermal stress on humans, especially in populated regions where dry-bulb temperature and relative humidity combine to make the heat feel even more oppressive. In places like Canada, Russia, and parts of Australia, the thermal stress index (TSI) can increase by over 6°C during these events.

Historical Trends and Future Risks

The study observed a notable rise from the year 2000 to 2020 in heatwave frequency across the globe, with sunny-dry heatwaves showing the most extensive increase, particularly in semi-arid regions. Approximately 67% of terrestrial areas have seen a doubling in sunny-dry heatwave days compared to earlier decades (1979–1999). Additionally, advective and adiabatic heatwaves have started to affect areas where they were not previously observed, such as Greenland and parts of northern Europe.

This rise in frequency and intensity means that more regions are exposed to the adverse effects of each heatwave type, with significant implications for water resources, food production, and public health. For example, regions that recently started experiencing sunny-dry heatwaves, like Central South America and western Europe, are facing increased risks to vegetation and crop yields.

What Can We Learn?

This research emphasizes the importance of understanding not just the intensity but the type of heatwave affecting each region. Detailed classifications like these allow for more targeted responses:

• Surface Energy Budget: One major takeaway is that analyzing the surface energy budget—how energy is absorbed, reflected, and emitted at Earth’s surface—can help us understand different heatwave types and predict their impacts more accurately.
• Localized Agricultural Planning: Knowing which heatwave types affect a region enables planners to choose crops that can better withstand specific conditions.
• Public Health Measures: Regions prone to advective heatwaves can prioritize health measures aimed at reducing thermal stress in vulnerable populations.
• Improved modelling: Integrating heatwave types into climate models helps forecast their occurrence and effects, allowing for more accurate impact assessments and proactive mitigation.

By recognizing the different types of heatwaves and their unique drivers, policymakers, scientists, and communities can design more effective adaptation strategies. Whether it’s adjusting agricultural practices to maintain crop yields or developing urban infrastructure to handle extreme temperatures, understanding these differences is crucial to building resilience in a warming world.

References

Tian, Y., Kleidon, A., Lesk, C., Zhou, S., Luo, X., Ghausi, S.A., Wang, G., Zhong, D. and Zscheischler, J., 2024. Characterizing heatwaves based on land surface energy budget. Communications Earth & Environment, 5(1), p.617.

Ghausi, S. A., Tian, Y., Zehe, E., & Kleidon, A. (2023). Radiative controls by clouds and thermodynamics shape surface temperatures and turbulent fluxes over land. Proceedings of the National Academy of Sciences, 120(29), e2220400120.

Tian, Y., Ghausi, S. A., Zhang, Y., Zhang, M., Xie, D., Cao, Y., … & Kleidon, A. (2023). Radiation as the dominant cause of high-temperature extremes on the eastern Tibetan Plateau. Environmental Research Letters, 18(7), 074007.