Global warming and more days of extreme heat are exacerbating the health risks of pregnancy, particularly among African-American women, according to new Stanford-led research.

The maternal mortality rate among all women in the United States is already the worst of any industrialized nation. And black women are three to four times more likely to die from pregnancy-related problems than white women.

“It is truly a crisis that in America, one of the wealthiest countries in the world, more women are dying from pregnancy or childbirth complications than in any other developed country,” said Maya Rossin-Slater, a core faculty member at Stanford Health Policy and a faculty fellow at the Stanford Institute for Economic Policy Research.

In a new working paper published by the National Bureau of Economic Research, Rossin-Slater and two other health economists underscore how little research is out there about the impact of rising temperatures on the health of mothers and their newborns.

Pregnant women, for example, are not able to regulate body temperature as efficiently as non-pregnant individuals due to the physiological changes they undergo during gestation. Heat exposure can alter blood flow in the placenta, which can weaken the placenta and lead to complications. And high heat can lead to other pregnancy complications, such as hypertension, preeclampsia and prolonged premature rupture of membranes.

“All of these issues can translate into women needing to be hospitalized during pregnancy and experiencing complications during childbirth,” wrote Rossin-Slater, an assistant professor of health research and policy at Stanford Medicine. Her co-authors are Jiyoon Kim, assistant professor of economics at Elon University, and Ajin Lee, an assistant professor of economics at Michigan State University.

The researchers said most of the discussion about maternal health focuses on the health-care system, but that other determinants of poor maternal health and racial disparities are much less understood, particularly when it comes to how the environment is impacting pregnancy.

So they launched what they believe is the first study to identify the causal effects of prenatal exposures to extreme temperatures on the health of the mothers themselves.

As the Earth Warms, So Does Exposure to Extreme Heat

Their paper focuses on an environmental factor that is becoming increasingly relevant due to the growing consensus that climate change is contributing to a gradual warming of the earth: exposure to extreme heat.

The researchers studied the effects of exposure to extreme temperatures during pregnancy on maternal and child hospitalizations, using inpatient discharge records from three U.S. states with different climates: Arizona, New York and Washington. Their data comes from the State Inpatient Databases from the Healthcare Cost and Utilization Project, including 2.7 million inpatient records of 2.7 million infants and 2.2 million mothers in those three states.

And to measure temperature exposure, the researchers obtained data from the National Oceanic and Atmospheric Administration (NOAA).

For every county in their data, the researchers calculated the average temperature for every month. Then for every given day in a specific month in that county, they looked at the historic average for how high or low that day’s temperature was relative to the overall temperature in that month in that county.

For example, a 90-degree day in Arizona in September would not be classified as extreme heat since it’s relatively common. But a 90-degree day in New York would be, since temperatures that high are much less common. They classified “extreme heat” as a given day when the temperature is more than three standard deviations (3SD) above that historic county mean.

Then, they compared the outcomes of women who are of the same race giving birth in the same county and calendar month, but in different years. These women are likely similar in terms of their demographics and socioeconomic status, but may be exposed to different temperatures during pregnancy. For example, consider a black woman giving birth in November 2011 in Queens County, New York, and a black woman giving birth in November 2012 in the same county. If there were a heat wave in Queens in the August 2012, then the latter woman is exposed to more extreme heat during pregnancy than the former. 

The economists found that each additional day with heat that is at least 3SDs — or substantially higher than the historic county-month average — during the second trimester of pregnancy increases the likelihood that a newborn is diagnosed with dehydration by .008 percentage points.

“Our results provide new estimates of the health costs of climate change and identify environmental drivers of the black-white maternal health gap,” they wrote. “Understanding the health consequences of this increase in extreme heat is critical information for discussions about the costs of climate change and the possible benefits of mitigating policies.”

The researchers found that each additional day of extreme heat exposure during pregnancy increases black women’s likelihood of hospitalization during pregnancy. Since black women on average are exposed to more extreme heat than white women — due to different residence patterns and access to mitigating technologies like air conditioning — extreme heat may contribute to exacerbating the already large gap in maternal health between black and white women.

Detrimental Consequences of Rising Temperatures

Scientists predict global average temperatures will continue to rise over the next 50 to 100 years as greenhouse gases continue to trap more heat in the Earth’s atmosphere. The U.N. Intergovernmental Panel on Climate Change last year warned that nations worldwide must quickly reduce fossil fuel use to keep the rise in global temperatures below 1.5°C by 2050. 

The panel also said the number of days with mean temperatures above 32°C in the average American county is forecasted to increase from about 1 to 43 days per year by 2070-2099.

That could have detrimental consequences for babies and mothers alike.

“Overall, our findings on infant health suggest that exposure to extreme heat during the second trimester increases the likelihood of the baby being dehydrated at the time of birth,” the researchers wrote. “This, in turn, appears to increase the likelihood of subsequent readmission to the hospital many months later for causes linked to dehydration.”

And these impacts are typically missed when researchers only measure infant health using more standard variables, such as birth weight.

The authors note dehydration is one of the leading causes of morbidity and mortality in children. Studies show that children under 5 years old who have an average of two episodes of gastroenteritis associated with dehydration per year leads to 2 to 3 million pediatric office visits and accounts for 10% of all pediatric hospital admissions in the United States. 

Experts believe black women are three- to four-times more likely to die from pregnancy-related causes due to lack of access to and the poor quality of health care, as well as clinicians not monitoring black women as closely — or actually dismissing their symptoms altogether.

“The fact that the adverse impacts on health during pregnancy are larger for black than for white mothers suggests that climate change may exacerbate the already large racial gap in maternal health,” the researchers said.

In a shack that now sits below sea level, a mother in Bangladesh struggles to grow vegetables in soil inundated by salt water. In Malawi, a toddler joins thousands of other children perishing from drought-induced malnutrition. And in China, more than one million people died from air pollution in 2012 alone.

Around the world, climate change is already having an effect on human health.

In a recent paper, Katherine Burke and Michele Barry from the Stanford Center for Innovation in Global Health, along with former Wellesley College President Diana Walsh, described climate change as “the ultimate global health crisis.” They offered recommendations to the new United States president to address the urgently arising health risks associated with climate change.

Bangladeshi children make their way through flood waters.

The authors, along with Stanford researchers Marshall Burke, Eran Bendavid and Amy Pickering who also study climate change, are concerned by how little has been done to mitigate its effects on health.

“I think it’s likely that health impacts could be the most important impact of climate change,” said Marshall Burke, an assistant professor of earth system science and a fellow at the Freeman-Spogli Institute for International Studies.

There is still time to ease — though not eliminate — the worst effects on health, but as the average global temperature continues to creep upward, time appears to be running short.

“I think we are at a critical point right now in terms of mitigating the effects of climate change on health,” said Amy Pickering, a research engineer at the Woods Institute for the Environment. “And I don’t think that’s a priority of the new administration at all.”

Health effects of climate change

Even in countries like the United States that are well-equipped to adapt to climate change, health impacts will be significant.

“Extremes of temperature have a very observable direct effect,” said Eran Bendavid, an assistant professor of medicine and Stanford Health Policy core faculty member.

“We see mortality rates increase when temperatures are very low, and especially when they are very high.”

Bendavid also has seen air pollutants cause respiratory problems in people from Beijing to Los Angeles to villages in Sub-Saharan Africa.

“Hotter temperatures make it such that particulate matter and dust and pollutants stick around longer,” he said.

In addition to respiratory issues, air pollution can have long-term cognitive effects. A study in Chile found that children who are exposed to high amounts of air pollution in utero score lower on math tests by the fourth grade.

“I think we’re only starting to understand the true costs of dirty air,” said Marshall Burke. “Even short-term exposure to low levels can have life-long effects.”

Low-income countries like Bangladesh already suffer widespread, direct health effects from rising sea levels. Salt water flooding has crept through homes and crops, threatening food sources and drinking water for millions of people.

“I think that flooding is one of the most pressing issues in low-income and densely populated countries,” said Pickering. “There’s no infrastructure there to handle it.”

Standing water left over from flooding is also a breeding ground for diseases like cholera, diarrhea and mosquito-borne illnesses, all of which are likely to become more prevalent as the planet warms.

On the flip side, many regions of Sub-Saharan Africa — where clean water is already hard to access — are likely to experience severe droughts. The United Nations warned last year that more than 36 million people across southern and eastern Africa face hunger due to drought and record-high temperatures.

Residents may have to walk farther to find water, and local sources could become contaminated more easily. Pickering fears that losing access to nearby, clean water will make maintaining proper hygiene and growing nutritious foods a challenge.

Climate change will affect health in all sectors of society.

All of these effects and more can also damage mental health, said Katherine Burke and her colleagues in their paper. The aftermath of extreme weather events and the hardships of living in long-term drought or flood can cause anxiety, depression, grief and trauma.

Climate change will affect health in every sector of society, but as Katherine Burke and her colleagues said, “….climate disruption is inflicting the greatest suffering on those least responsible for causing it, least equipped to adapt, least able to resist the powerful forces of the status quo.

“If we fail to act now,” they said, “the survival of our species may hang in the balance.”

What can the new administration do to ease health effects?

If the Paris Agreement’s emissions standards are met, scientists predict that the world’s temperature will increase about 2.7 degrees Celsius – still significant but less hazardous than the 4-degree increase projected from current emissions.

The United States plays a critical role in the Paris Agreement. Apart from the significance of cutting its own emissions, failing to live up to its end of the bargain — as the Trump administration has suggested — could have a significant impact on the morale of the other countries involved.

“The reason that Paris is going to work is because we’re in this together,” said Marshall Burke. “If you don’t meet your target, you’re going to be publicly shamed.”

The Trump administration has also discussed repealing the Clean Power Plan, Obama-era legislation to decrease the use of coal, which has been shown to contribute to respiratory disease.

“Withdrawing from either of those will likely have negative short- and long-run health impacts, both in the U.S. and abroad,” said Marshall Burke.

Scott Pruitt, who was confirmed today as the head of the Environmental Protection Agency (EPA), is expected to carry out Trump’s promise to dismantle environment regulations.

Despite the Trump administration’s apparent doubts about climate change, a few prominent Republicans do support addressing its effects.

Secretary of State Rex Tillerson, the former chairman and CEO of Exxon Mobile, supports a carbon tax, which would create a financial incentive to turn to renewable energy sources. He also has expressed support for the Paris Agreement. It is possible that as secretary of state, Tillerson could help maintain U.S. obligations from the Paris Agreement, though it is far from certain whether he would choose to do so or how Trump would react.

More promising is a recent proposal from the Climate Leadership Council. Authored by eight leading Republicans — including two former secretaries of state, two former secretaries of the treasury and Rob Walton, Walmart’s former chairman of the board — the plan seeks to reduce emissions considerably through a carbon dividends plan.

Already an issue, malnutrition will increase with droughts in Sub-Saharan Africa.

Their proposal would gradually increase taxes on carbon emissions but would return the proceeds directly to the American people. Americans would receive a regular check with their portion of the proceeds, similar to receiving a social security check. According to the authors, 70 percent of Americans would come out ahead financially, keeping the tax from being a burden on low- and middle-income Americans while still incentivizing lower emissions.

“A tax on carbon is exactly what we need to provide the right incentives and induce the sort of technological and infrastructure change needed to reduce long-term emissions,” said Marshall Burke.

Pickering added, “This policy is a ray of hope for meaningful action on climate.”

It remains to be seen whether the new administration and congress would consider such a program.

What can academics do to help?

Meanwhile, academics can promote health by researching the effects of climate change and finding ways to adapt to them.

“I think it’s fascinating that there’s just so little data right now on how climate change is going to impact health,” said Pickering.

Studying the effects of warming on the world challenges traditional methods of research.

“You can’t create any sort of experiment,” said Bendavid. “There’s only one climate and one planet.”

The scholars agree that interdisciplinary study is a critical part of adapting to climate change and that more research is needed.

“If ever there was an issue worthy of a leader’s best effort, this is the moment, this is the issue,” said Katherine Burke and her colleagues. “Time is short, but it may not be too late to make all the difference.”