The Health Risks at 2.0°C Climate Warming Are Higher Than the Risks at 1.5°C of Warming

Introduction: The signatories to the Paris Agreement under the United Nations Framework Convention on Climate Change agreed to “hold the increase in the global average temperature to well below 2.0°C above pre-industrial levels, recognizing that this would significantly reduce the risks and impacts of climate change.” The question to the scientific community is what is the magnitude and pattern of risks at these increases in global mean surface temperature, to better understand which risks would likely be avoidable by additional mitigation and which would still need to be managed.

Methods: We identified all studies cited in the Human Health chapter of the Intergovernmental Panel on Climate Change 5^th Assessment Report (2014) and published subsequently that projected the health risks of temperature-related mortality (n=28), undernutrition (n=4), and vector-borne diseases (n=45). For each study, we summarized the health outcome metric, the temporal baseline and time periods of interest, the climate models (GCMs) and emission scenarios used, and the projected impacts. Because few studies reported impacts by temperature change, we developed a baseline period against which to measure 1.5^oC and 2.0^oC temperature increases. Rather than define a specific pre-industrial baseline period (which is challenging due to the different historical starting points among GCMs), within each GCM projection the decade 2010-2019 was defined as the baseline. To determine when 1.5^oC and 2.0^oC above preindustrial conditions were reached for a given temperature projection, we moved the 10-year window forward one year at a time, starting with 2011-2020, until the projected global average temperatures in the window were 0.5 ^oC and 1.0 ^oC above the baseline. In addition to identifying the decades for which 1.5 ^oC and 2.0 ^oC above preindustrial conditions were reached for each case, the same approach was used to estimate the magnitude of global average temperatures increase in the last two decades of the 21^st century, 2080-2089 and 2090-2099.

Results: Projected heat-related mortality and projected hazardous heat conditions are projected to increase with the degree of temperature change. Assuming a linear relationship between high ambient temperature and mortality, the magnitude of the health impacts of 2.0°C are projected to be greater than 1.5°C, with the extent varying by region, presumably because of average temperatures (e.g. risks are higher in regions with cooler average temperatures), population vulnerability, the built environment, access to air conditioning, population acclimatization, and other factors.

Recent publications support the conclusions that climate change will negatively affect childhood undernutrition, particularly stunting, through reduced food availability, and will negatively affect undernutrition-related childhood mortality and disability-adjusted lives lost (DALYs), with the largest risks in Asia and Africa. The projected health risks are projected to be greater at 2.0° vs 1.5°C warming.

Recent projections of the potential impacts of climate change on malaria globally and for China, Asia, Africa, and South America confirm that weather and climate are among the drivers of the geographic range, intensity of transmission, and seasonality of malaria, and that the influences of temperature and precipitation are nonlinear. Most projections suggest the burden of malaria could increase with climate change because of a greater geographic range of the Anopheles vector, longer season, and/or increase in the number of people at risk, with larger burdens with greater amounts of warming, with complex regional patterns.

Projections of the geographic distribution of Aedes aegypti and Ae. Albopictus, and of the prevalence of dengue fever, generally conclude there will be an increase in the number of mosquitos and a larger geographic range of dengue in the 2030s, with regional differences. The risks are projected to increase with greater warming and under higher greenhouse gas emission pathways.

Projections in North America and Europe suggest a latitudinal and altitudinal expansion of regions climatically suitable for West Nile Virus transmission, particularly along the current edges of its transmission areas, and extension of the transmission season, with the magnitude and pattern of changes varying by location and degree of warming.

Most projections conclude that climate change will expand the geographic range and seasonality of Lyme and other tick-borne diseases in North America and Europe. The projected changes are larger with greater warming and under higher greenhouse gas emission pathways.

Conclusions: Greater degrees of warming will increase the magnitude of health risks to which communities and health systems will need to prepare and manage. Rapid reductions of greenhouse gas emissions would mean few health risks to manage past mid-century.