Global Emissions and Local Deforestation Are Combining to Create Dangerous Levels of Heat Stress in the Tropics

Average global temperatures have risen by 1.1 degrees C (2.0 degrees F) since pre-industrial times, and we’re already seeing temperature extremes that threaten human health. Heat waves killed hundreds in North America in 2021 and thousands in Europe in 2022. And it’s only poised to worsen unless emissions drop precipitously.

But in some parts of the world, it’s not just climate change that’s causing dangerously high heat.

A recent WRI report shows that in addition to fueling global climate change through CO2 emissions, deforestation in the tropics has acute local effects on climate, resulting in increased average and extreme local temperatures. The local warming effects of deforestation in the tropics are comparable to those from greenhouse gas-driven warming, creating a double-whammy effect on temperatures that threatens public health and economies.

While climate mitigation and adaptation strategies account for emissions-driven warming in the tropics and elsewhere, few policies acknowledge additional heat driven by local deforestation. Here, we unpack the combined effects of local and global warming in the tropics and their implications for the people who live there.

2 Sources of Tropical Warming: Global Greenhouse Gases and Local Deforestation
A study published in Nature estimates that 30% of the world’s population is already exposed to combinations of heat and humidity that exceed what’s safe for the human body at least 20 days a year. By 2100, under a climate scenario that limits warming to 2 degrees C (3.6 degrees F), nearly half the global population will be exposed to climatic conditions that are potentially deadly for more than 20 days per year.

These impacts will be particularly acute in the tropics, where many communities will experience unsafe conditions for several months a year. Since daily temperatures are already higher and less variable near the equator, it doesn’t take much of an increase to exceed comfortable temperature and humidity thresholds for the human body. And given that a disproportionate share of low- and lower-middle income countries, including those with the highest number of people in extreme poverty, are clustered around the equator, global warming is expected to disproportionately impact low-income populations.

Importantly, these estimates do not yet take into account the additional localized warming effects of deforestation.

At all latitudes, from the tropics to the boreal zone, forests help stabilize the local climate by reducing extreme temperatures and maintaining rainfall patterns. Tropical deforestation poses especially large risks to human well-being because tropical forests provide local cooling benefits to regions already expected to be significantly affected by extreme heat.

Through evapotranspiration, for example, trees help convert surface and ground water into atmospheric moisture, serving as a natural air conditioning system. Uneven forest canopies cause wind turbulence that can lift heat and moisture away from Earth’s surface. These processes also play a role in increasing cloud cover over tropical forests, which in turn reflects more sunlight, facilitating further cooling.

While estimates vary, tropical deforestation increases the annual local average temperature by approximately 1 degree C (1.8 degrees F). The impacts, however, are even more significant when one looks at extremes: Deforestation can lead to an average increase of 4.4 degrees C (7.9 degrees F) warming in daily high temperatures in the tropics. In one study, researchers found that by 2100, the heat stress caused by continued widespread deforestation in the Brazilian Amazon would be comparable to what’s expected under the worst climate change scenarios.

While the temperature increases due to greenhouse gas-driven warming happen gradually, those due to forest clearance happen abruptly. And it’s not just people living right next to a deforested patch who are affected. Local temperature effects of deforestation increase with the scale of the deforested patch, and have been detected up to 50 kilometers away.

The Impacts of Higher Temperatures on Rural Workers
What happens to the human body in extreme heat? When body temperature rises above its baseline of 37 degrees C (98.6 degrees F), blood thickens, forcing the heart to work overtime, causing damage to it and other organs. Once the body is unable to cool itself by sweating –– which can happen in high humidity when the air is already saturated with moisture –– dehydration and other symptoms of heat exhaustion such as nausea, dizziness and difficulty breathing can occur. If left untreated, an individual may suffer heat stroke, organ failure, neurological damage and, potentially, death. Heat stress may also contribute to kidney disease.

Extreme heat can also affect mental function. A 2020 study of rural agricultural workers in Indonesia found that those laboring in hotter deforested areas scored lower on general cognitive assessments and memory tests than workers located in forested areas. Researchers attributed the differences in test scores primarily to heat exposure. Diminished cognitive functioning due to heat exposure can lead to a number of other negative outcomes, such as increasing workers’ risk of injury.

Outdoor workers in the tropics, such as farmers and agricultural workers, are particularly vulnerable to extreme heat given that many lack access to cool shelters and their jobs require working outside even in hot conditions. This vulnerability is compounded by the fact that many jobs on the frontiers of deforestation tend to be informal and thus unprotected by government safety regulations and enforcement mechanisms.

Lack of worker safety protections manifests in a variety of ways. To give one example: A 2019 study of rural workers in Indonesia found that more than 40% of study participants working in open areas didn’t have access to water when they were working. Yet temperatures in open areas were up to 8.3 degrees C (14.9 degrees F) warmer relative to forested areas, exposing workers to temperatures and humidity levels well above human well-being thresholds for up to 6.5 hours a day.

Agricultural workers in hot temperatures also face the risk of increased pesticide exposure and poisoning. Negative cognitive impacts due to heat exposure can diminish workers’ ability to be proactive about safety. Agricultural workers are also less likely to wear protective equipment during hot days; those who do wear it will be more vulnerable to heat stress. The fact that sweating can increase the absorption of chemicals through the skin further increases the risk of pesticide poisoning on hot days.

Deforestation in the Brazilian Amazon: The Impacts of Heat Stress on Health and the Economy
Brazil, home to the majority of the Amazon rainforest, is a good case study for considering the health and associated economic impacts of increased exposure to extreme heat. Situated in the tropics, Brazil is highly affected by global warming and is a site of significant deforestation.

The extent of deforestation in the Amazon is estimated to be around 17%, with deforestation in the Brazilian Amazon around 20%. Researchers believe that once Amazonian deforestation reaches 20-25%, the effects could be irreversible as the landscape shifts from forest to savanna.

Let’s look at what the compounding impacts of local deforestation-related warming and climate change may mean for future Brazilians. Under continued deforestation and a moderate warming scenario where emissions peak by mid-century and then begin to decline, more than 6 million Brazilians will likely experience heat-related risks to their health by 2100. (Here, risk is defined by temperature, relative humidity and wind speed conditions that exceed the thresholds that are considered safe for the human body for at least an hour daily.) In a more dire warming scenario, where emissions continue to rise through the end of the century alongside continued deforestation, more than 11 million Brazilians are estimated to be at risk by 2100 –– approximately 5% of Brazil’s current population.

For either climate scenario, 40% or more of those affected would be in highly vulnerable populations: communities that lack sufficient living conditions in terms of health, economic development, infrastructure, access to education and other factors.

Not only do rising temperatures in the tropics endanger human health; they can also have significant economic impacts. A study by the International Labor Organization estimates that even if global temperature rise is held to 1.5 degrees C (2.7 degrees F), Brazil would still lose the equivalent of 850,000 full-time jobs by 2030 because of reduced working hours due to heat stress exposure. Deforestation-driven warming has the potential to exacerbate these economic impacts by further reducing the number of safe working hours in surrounding areas.

In a 2021 study focusing on two states in Brazil, Mato Grosso and Pará, where more than half of Brazilian deforestation occurred between 2008 and 2019, researchers found a strong relationship between large-scale deforestation and lost working hours. In deforested areas, 45% of workers lost a half hour or more of daily safe work time versus less than 5% of workers in forested areas.

Job and income loss will translate to additional negative impacts on human health and, for those who are employed, may mean added pressure to work in hazardous conditions. The more tropical forests are kept intact, the less severe the inevitable economic impacts of global temperature rise will be for tropical countries.

There is reason for optimism that Brazil may be able to avoid some of these negative health and economic impacts. On October 30, 2022, former President Luiz Inácio Lula da Silva defeated incumbent President Jair Bolsonaro to become the next Brazilian president. Lula committed to reverse Bolsonaro’s harmful environmental policies and has already made moves in that direction. If the past is a guide — and if Lula can overcome political opposition — the future of the Amazon looks more promising: Deforestation rates declined by more than 70% during Lula and his successor’s presidencies from 2004-2016.

The advent of a new administration in Brazil also provides an opportunity to integrate the public health implications of deforestation into the country’s climate and land-use policies.

3 Pathways to Reduce Deforestation-related Heat Stress
What can be done to mitigate the negative public health impacts of deforestation-driven warming in the tropics? Here are three ways countries can act:

1) Integrate deforestation-induced local warming into national climate plans and strategies.
Most countries, as signatories to the Paris Agreement, have created a national plan outlining their strategies to limit emissions and adapt to climate change. These plans, known as “nationally determined contributions,” or NDCs, are updated every five years, but few if any have considered the effects of deforestation-induced local warming.

For example, Brazil last updated its NDC in March 2022. While it mentions the impact global temperature rise has on productivity and employment in the agricultural sector, there is no mention of the impacts of local deforestation-related temperature rise on the economy or on the Brazilian people. Given that Brazil’s NDC states a desire to reduce public health vulnerabilities, it will be important for the next iteration to include the local cooling benefits provided by tropical forests.

2) Address risks of deforestation-induced local warming in sectoral policies, practices and regulations.
Alongside their roles in national-level climate change planning, public health agencies, worker safety regulators and agricultural ministries can protect individuals in the tropics from the dangers of deforestation-driven warming.

In Brazil, for example, the Ministry of Health could provide tools and alert systems to identify when and where individuals face significant risks due to climatic conditions, as well as advisories and protective measures for those in occupations with higher risk of heat stress, such as agricultural and construction workers. Such systems would enable more rapid responses by the health sector and the public to the risks posed by extreme heat. Additionally, the Ministry of Health could spearhead education campaigns that inform the public more broadly of the dangers of heat exposure, especially near recently deforested sites.

The Brazilian Regulatory Standards could mandate additional protections for workers, such as requiring employers to provide access to water and shade, as well as frequent monitoring of worker temperature and vital signs on hot days. It would then fall to the Ministry of Labour and Employment to ensure that these improved workplace standards are implemented. Since more than 40% of employment in Brazil is informal and therefore not covered by labor regulations, additional safeguards will need to be devised that help protect these workers.

Finally, the Ministry of Agriculture, Livestock, and Food Supply could work with government officials, including the Ministry of Health and the Ministry of Environment, to ensure that agricultural development policies are well-balanced with human health objectives and Brazil’s climate plan, as deforestation is often associated with agricultural expansion.

3) Raise awareness of the national and local risks of deforestation.
For most policymakers, the climate-related risks of deforestation are understood to be limited to those related to greenhouse gas emissions and their contribution to the gradual warming of the entire planet, and thus the business of climate negotiators. The impacts of the more immediate — and equally severe — effects of deforestation on local temperatures are relevant to the mandates of other public and private actors, including those charged with protecting public health and worker safety. Scientists and civil society advocates should target their outreach efforts to such non-traditional audiences.

Different agencies and stakeholder groups need to break down silos and align their goals to promote, above all, citizens’ well-being. For example, climate adaptation planning to protect public health needs to involve experts on the impacts of land-use change on heat stress exposure, in addition to those who track the spread of disease vectors with rising global temperatures.

It is important that farmers, companies, agricultural ministries, public health officials and worker safety agencies come to realize that they, too, are stakeholders in forest protection.