Last winter’s record cold made it hard to think about global warming, but the horrific winter was the “weather on steroids” that climate scientists have been predicting for decades in which hotter days get hotter and cold spells are even more intense.

Beyond images of emaciated polar bears and drought-cracked lakes, however, there remains a major part of climate change’s impact that the media have neglected, yet it is the one that may have the most immediate and profound consequence for our lives: how rising temperatures, higher carbon-dioxide levels and the corresponding changes in ecosystems will have a serious effect on our health.

A recent report in the British medical journal The Lancet noted that “climate change is the biggest global health threat of the 21st century.”

In the coming decades, we’ll be living with higher levels of ozone pollution in the air we breathe, killer heat waves, more uncontrolled outbreaks of deadly bug-borne infectious diseases as vectors migrate to newly warm habitats, and fallout from drought-driven agricultural collapse and from increasingly frequent extreme weather events — floods, hurricanes, fires. Because hot air holds more water, we’ll have more torrential rains, more ferocious hurricanes and more dry spells because of heat-induced changes in rainfall patterns. All these changes translate into increasing rates of ills such as asthma, allergies, severe respiratory infections, heart and lung disease, cancer, infectious diseases and even dementia and depression.

There is also the collateral damage from the harsher climate, such as the debilitating injuries and deaths, the dislocation and loss of social cohesion, and the lack of continuity in health care in the aftermath of weather-related calamities. What happened after Hurricane Katrina and superstorm Sandy — when parts of the public health system collapsed and thousands of people went without needed medical care, sometimes for months, leading to much higher rates of disability and mortality — provides a chilling glimpse into a future when storms of this magnitude become commonplace.

The rise of dengue fever provides a window into how a hotter planet will promote the spread of infectious disease, and how warmer weather triggers chain-reaction changes in delicately calibrated ecosystems. Over the past half-century, the incidence of dengue fever has spiked 30-fold, according to the World Health Organization, and it now causes an estimated 100 million infections annually in more than 100 countries, especially in densely populated and developing megacities in the tropical belt.

Dengue also has traveled northward and taken up residence in the United States, and is now endemic in Puerto Rico and Florida, which had an outbreak last summer. In the past decade, more than 10,000 cases have been reported here, according to the Centers for Disease Control, which also found that hospitalizations for dengue fever tripled during that same period.

Rising temperatures create an incubator for transmission because the mosquitoes that spread dengue are now able to survive in places that were once too cold. Since the 1960s, two types of mosquitoes that can transmit dengue — Ae. aegypti and Aedes albopictus — have expanded their range to at least 28 states, even as far north as New York and New Hampshire, making the 173 million Americans who live in these areas vulnerable, according to a July 2009 report by the Natural Resources Defense Council. When the weather is warmer, the breeding cycles of the mosquitoes shorten, which means the bugs can reproduce multiple times instead of once or twice. Heat also speeds up the incubation of the dengue virus inside the bugs, so it becomes infective much faster, which means it has a longer window in which to sicken someone during its three-to-four-week life span. Plus, female mosquitoes bite more frequently when it’s hotter, boosting the capacity to transmit the virus.

And this is just the beginning. If we don’t curb greenhouse gas emissions, we are on the cusp of an increasingly unhealthy future. Last year, we hit 400 ppm (parts per million) of carbon-dioxide for the first time in about 3 million years, based on readings from the Mauna Loa Observatory in Hawaii. The last time carbon-dioxide hit these levels in the atmosphere, temperatures jumped 3 to 5 degrees Fahrenheit and sea levels were at least 30 feet higher, which would flood 25 percent of the U.S. population, mainly on the eastern and Gulf coasts.

If we do nothing to curb emissions, the U.S. is on track for temperature increase of 4 to 11 degrees Fahrenheit by century’s end and carbon-dioxide ppm counts up to 1,000, choking cities in a cloud of smoke that would far surpass Beijing on its worst air days.

Already, the heat-trapping carbon dioxide that’s emitted from tailpipes and factories collects over cities, creating carbon-dioxide “domes” that shroud the urban cores in toxic clouds of pollutants. In cities like New York, Phoenix and Baltimore, ambient carbon-dioxide parts per million (ppm) levels can spike into the 400s, 500s, and 600s, which is what will become the norm in the next few decades, according to climate models. A 2010 Stanford University study found that these domes exacerbate pollution’s harmful health effects, and may cause 1,000 deaths a year.

It’s only going to get worse as the planet continues to warm. Two of the chief culprits behind asthma, allergies and respiratory illnesses — air pollution and smog — intensify as temperatures rise. Children may be the ones who suffer the most from the effects of increasingly bad air, according to results of a long-term University of Southern California study on the link between chronic exposure to air pollution from freeway traffic and respiratory illnesses.

After a decade of meticulous monitoring, USC researchers found that the development of the lungs of children who lived near highly trafficked corridors was stunted, about 20 percent smaller than normal, which greatly impaired their normal functioning. These youngsters also suffered from asthma at significantly higher rates — those living within a quarter of a mile from a freeway had an 89 percent higher risk of asthma than kids more than a mile away — their asthma symptoms were worse, and their odds of ending up in the hospital emergency room was triple that of youngsters in more affluent areas.

Growing scientific evidence also indicates that hellish hot spells like the one that gripped the nation last summer have become far more likely to occur. Within the next 30 years, exceptionally long heat waves could become the norm, particularly in the American West, according to one 2010 Stanford University study. Overall, the frequency of so-called extreme weather events — when temperatures climb past 100 degrees — could triple over the remainder of this century, research by University of Miami scientists showed, resulting as many as 150,000 deaths between now and 2100.

In the absence of mitigation strategies, we’re going to live sicker and die quicker on a hotter planet. “We’re going to see incremental changes in the next 5 or 10 years but that might not compare to what we’re going to see in the next 30 or 40 years,” says Dr. John Balbus, a senior policy analyst who leads the climate change and health effort at the National Institute for Environmental Health Sciences. “Scientists don’t want to be alarmist, but they have systematically underestimated the threat.”

Linda Marsa is author of “Fevered: Why a Hotter Planet Will Hurt Our Health and How We Can Save Ourselves.” Email at lmarsa@sbcglobal.net. She wrote this for the Free Lance-Star in Fredericksburg, Va.; it was distributed by MCT Information Services.