A draft summary of the Intergovernmental Panel on Climate Change's latest report on the impacts of global warming leaked into the blogosphere last Friday. The draft highlights concerns ranging from melting sea ice to diminishing crop yields to health dangers from hunger and heat waves. What it does not address, however, is the added possibility that climate change could magnify the havoc wrought by long-lasting and pervasive toxic chemicals.
Two of the greatest threats to global health, some scientists say, could be closely connected.
"We just barely missed the deadline for the IPCC," said Michael Hooper, a research biologist at U.S. Geological Survey's Columbia Environmental Research Center in Missouri.
Hooper referred to a series of studies he and other scientists published in January, after the cut-off for consideration by the climate group in its fifth assessment report (which could change before the final version is due in March). The new studies highlight how global warming may affect the movement and levels of chemicals such as organochlorine pesticides in the environment, as well as how a changing climate might weaken the ability of animals and humans to tolerate those chemicals.
The reverse is also addressed: Greater chemical exposures could hamper a polar bear's or human baby's ability to handle extreme temperatures, severe storms, lack of food or other hazards of climate change.
"Climate change could also make these compounds available in a more toxic form, for a longer period of time or at a higher concentration in the body," added Hooper.
The science is only just emerging and carries with it a lot of uncertainty, open questions and what Hooper called "layers upon layers upon layers of complexities."
"The issue of health effects of [persistent organic pollutants, or POPs] coupled with climate change is a complex, tricky issue," said Ana Priceputu, a program officer with the United Nations Stockholm Convention.
"The mechanisms of actions in humans are still not well characterized," she added. "On top of that, humans are exposed to not only one substance but to various sets of substances, and we're adding climate stimuli such as heat waves."
Several organochlorine pesticides, including DDT, hexachlorobenzene and mirex, were among the first POPs listed under the Stockholm Convention. Enacted in 2004, the international treaty aims to protect the environment and public health from toxic chemicals that accumulate in the environment, travel great distances and concentrate up the food chain. Many of the same chemicals were widely banned, or at least heavily restricted, decades ago in the U.S., but still linger in the soil, waterways and residents' bodies. The U.S. has not yet ratified the convention.
In October, the convention adopted new guidance to account for climate change in its work, based on the group's 2011 review of global warming's effects on the dynamics and toxicity of POPs.
"This stuff is just starting to get some traction," said Hooper.
Climate change, as several documents now explain, may unleash legacy organochlorines from melting ice, water and soil.
The tendency of chemicals in the ground or water to become airborne rises exponentially with increasing temperatures. Heavy rainfalls, also predicted with climate change, could wash organochlorines previously bound to soil into lakes and rivers, where they could then enter the food chain and ultimately our bodies. Global-warming enhanced erosion, too, could free organochlorines from the soil, while sea level rise could inundate contaminated land and waste sites, releasing still more.
Authors of the Stockholm Convention report urgently recommend that countries clean up legacy chemicals from dumps, soil, sediments and other places where chemicals may have settled during their decades of use.
Meanwhile, more organochlorine pesticides are still entering the environment.
The convention allows some exemptions to its bans, such as the use of DDT to combat malaria-carrying mosquitoes -- a war predicted to intensify as climate change expands the disease's range and speeds its spread.
Use of other organochlorine pesticides to control agricultural pests is also expected to change with the climate, as regions suitable for growing crops shift. These same chemicals may even be newly employed in pipes, sewage systems and waste treatment to combat microbes in northern regions, where colder temperatures have historically rendered those microbes inert, according to Roland Kallenborn, an author of several studies on the issue at the Norwegian University of Life Sciences.
Kallenborn said that the current understanding of climate-chemical interactions suggests a "significant change of exposure," particularly for indigenous Northern populations.
While he added that such concerns will be primarily regional and "rather small compared to the global issues associated with climate change," Kallenborn and other scientists agree that the emerging chemical-climate connection warrants further attention from the IPCC, the world's foremost authority on climate change.
The IPCC has focused on the "short-lived climate forcers" such as ozone, methane and particle concentrations, according to an emailed statement from the Danish authors of a separate study on organochlorine pesticides being released from warming oceans and melting ice.
Overlooked, they suggested, are the "cascade effects of climate change" on longer-lived chemicals such as POPs. "In our opinion," the authors said, "it is important that IPCC reserves a paragraph for such effects."
Exposure to organochlorine pesticides -- apart from any additional possible risks from climate change -- has already been linked to a number of human health problems, including birth defects, cancer and diabetes. New research continues to add concerns.
A study published on Monday found evidence that children exposed in the womb to two organochlorine pesticides, hexachlorobenzene and PCBs, were more likely to develop asthma.
On Tuesday, a separate new study hinted at a health risk that could be tied to the hormone-disrupting potential of the chemicals. Legacy organochlorines pesticides mirex and beta-hexachlorocyclohexane were more commonly found in the blood of study participants with endometriosis than those without the condition. Other organochlorines tested showed no association.
Endometriosis, which affects up to 10 percent of reproductive-aged women, is known to be linked to estrogen levels; and organochlorines have estrogenic properties.
"Our findings highlight the persistence of legacy pesticides," said lead author Kristen Upson, who was a pre-doctoral research fellow in epidemiology at the University of Washington when the study was conducted. "Past use may affect current generations of reproductive-age women."
It's no coincidence that organochlorine pesticides persist. They were designed to last, which is in large part what makes them so effective as pesticides. And even as they eventually degrade into lower and lower concentrations, more emerging science suggests they could still pose serious health risks, especially to vulnerable individuals like a developing fetus or young child.
"The traditional way for thinking about chemicals followed the model that increasing exposure increased harm," added Upson, now a postdoctoral fellow at the National Institute of Environmental Health Sciences. "We now understand with endocrine disruptors that chemicals can have different effects at different levels."
Organochlorines' effects may even extend beyond their presence in the environment, according to another study published in October. Researchers found that DDT may have played a role in high rates of obesity among rats three generations after exposure.
If the world were to stop emitting greenhouse gases today, the IPCC draft report notes, the planet is still locked into at least some warming that brings with it wide-ranging effects. The World Meteorological Organization announced this week that greenhouse gas emissions reached new records in 2012.
At least some exposure to organochlorine pesticides is also more or less unavoidable.
"Climate change is going to mobilize these POPs, so they are more likely to be encountered in our lives," said Michele La Merrill, an environmental toxicologist at the University of California Davis.
"Since many of these chemicals are stored in animal fat," she added, "regardless of whether or not you're talking about climate change, one of the best things to do is to eat lower on the food chain."