Population growth, now at roughly 78 million extra people per year, is the don't-go-there zone of modern environmentalism and political discourse.

But let's go there for the moment: The biodiversity crisis. The water crisis. The climate crisis. Lurking behind all these crises is at least one shared factor: human population. Species extinction? Think land clearing for agriculture to feed a growing population of 6.8 billion people. Water? The majority of water goes directly to growing that same food supply. And giving a helping hand to all these other crises as a result of all the fossil fuel burning needed to power our lives and lift billions out of poverty: anthropogenic climate change.

Temperatures on the Eastern seaboard have risen to the high 80s and low 90s in recent days, 20 to 30 degrees Fahrenheit above normal for April in the region. Here in New York City, where Scientific American's offices are located, we may break the record high of 90 degrees Fahrenheit on this date set back in 1990. But as the temperature climbs in the Northeast and summer wilt sets in before trees have even budded out, it's worth remembering that weather is not climate.

Weather is the day-to-day temperature, humidity or precipitation that determines whether you'll wear your spring coat or strip down for summer. Climate is the overall combination of all these elements over a long period of time.

Temperature records kept since the 19th century reveal that global average temperatures are inexorably creeping up, a phenomenon dubbed climate change. The cause? Increasing levels of greenhouse gases, most commonly carbon dioxide, in the atmosphere, which trap heat that would otherwise radiate back out to space, like a smothering blanket.

Today marks the 39th annual Earth Day, an idea hatched by Wisconsin Senator Gaylord Nelson in 1969 to "shake up the political establishment and force this issue into the national agenda," according to the Earth Day Network, a nonprofit that helps organize the day.

But way back before global warming was a household term and canvas totes were a fashionable alternative to shopping bags, environmental supporters started with the basics: recycling, energy use, pesticides and population growth, to name a few.

So how much have actions and attitudes about saving the earth changed since then? Mark Fischetti, managing editor of Scientific American Earth 3.0 magazine, reflects that, "Back in the '70s, Earth Day was kind of this quirky, one-day grassroots event. It raised a little awareness, but the next day it was gone… Now it's on the radar every single day, it's in the headlines every single day."   

There are some 82,000 chemicals used commercially in the U.S., but only a fraction have been tested to make sure they're safe and just five are regulated by the U.S. Environmental Protection Agency (EPA), according to congressional investigators. But a government scientist says there's no guarantee testing actually rules out health risks anyway.

The basic premise of safety testing for chemicals is that anything can kill you in high enough doses (even too much water too fast can be lethal). The goal is to find safe levels that cause no harm. But new research suggests that some chemicals may be more dangerous than previously believed at low levels when acting in concert with other chemicals.

"Some chemicals may act in an additive fashion," Linda Birnbaum said this week at a conference held at the Columbia Center for Children's Environmental Health at Columbia University. "When we look one compound at a time, we may miss the boat."

The fields of space and climate science are growing ever more closely entwined: Japan launched a new satellite to monitor greenhouse gases late last week, and NASA is set to launch its own Orbiting Carbon Observatory next month. But what about all the nasty fumes and gases spewed by the boosters needed to shoot those climate watchdogs into orbit?

A California company has a solution to shrink the ecological footprint of space exploration, but it remains to be seen whether it can or will be applied to real spaceflight: biodiesel-powered rockets. Flometrics, based in Carlsbad, Calif., earlier this month conducted a ground rocket-engine test of biodiesel (the "same stuff people put in their cars," according to company founder Steve Harrington) alongside RP-1, a standard rocket-grade kerosene fuel, and found them of almost equal fortitude. (The biodiesel delivered about 3 percent less thrust than the RP-1, according to Flometrics.) Biodiesel, a liquid fuel derived from vegetable oil or animal fat, has already been used to power a cross-country jet flight.

Reproductive health and enviro activists are fuming over two more last-minute rule changes by the outgoing Bush administration: a new reg that allows heathcare workers to nix treatments to which they have moral objections, and another one that bars regulators from taking into consideration a power company's climate change–causing greenhouse gas emissions when applying for a license to build new coal-fired plants.

Both rules are set take effect a month from now—just hours before Pres. Bush vacates the White House and President-elect Barack Obama is sworn in to office on Jan. 20.

Data from the U.S. Centers for Disease Control and Prevention (CDC) indicates that humans carry phthalates—chemicals used as softeners in plastics and found in everything from pill coatings to nail polish—around in their bodies. A growing number of studies, primarily in rats, show that phthalates cause male reproductive problems—infertility, decreased sperm count, malformation—and can cross the placenta. As a result, the European Union has banned some of them and consumer advocate and environmental groups have called for the U.S. government to do the same.

Today, an advisory panel of scientists, commissioned by the Environmental Protection Agency (EPA), released a report recommending that the chemicals be assessed as a group for potential risks as soon as possible.

Is your neighborhood using? Researchers from Oregon State University and the University of Washington have devised technology that analyzes what’s been flushed down the toilet to measure how many speed freaks and coke heads you’ve got living down the street.

A report published in the  Dec. 15 edition of the American Chemical Society journal Environmental Science & Technology describes a new test that uses standard chemical analytical methods to look at  what  stuff makes its way through the municipal sewer systems to wastewater treatment plants. There, the test can measure levels of drugs including illegal substances like crystal methamphetamine.  Unlike previous methods, the technique does not require expensive and time-consuming sample preparation, making it a practical for comparing drug use in different regions.

Among the many pressing issues that President-elect Barack Obama will face when he takes office in January is climate change, which he has called an “immediate threat” and warned has made Earth a “planet in peril.” In an effort to prevent and reverse the problem, he supports a so-called cap-and-trade scheme similar to one now in effect in the U.S. Northeast and the European Union.

Under such a plan, the government sets an overall limit on the amount of pollution allowed and polluters, such as power companies, are sold or given permits to pollute. Those who emit less pollution thanks to a new wind farm, for example, can then sell their excess pollution permits to other companies struggling to meet their quotas. That ensures that the industry stays within the overall emission limit, which declines over time.

The long-term effects of the 9/11 attacks aren’t merely existential. Whether the collapse of the Twin Towers and exposure to the stew of dust and chemicals caused disease, and the emotional toll it took on witnesses, are scientific questions, too.

New estimates suggest that of the more than 400,000 people who were directly exposed to the strikes, 35,000- to- 70,000 developed post-traumatic stress disorder (PTSD), and 9,700-to-2,000 people experienced serious psychological distress. Some 3,800 to 12,600 people may have developed asthma, New York City epidemiologists report in this month's Journal of Urban Health.