On Earth Day 2013, a planetary report card on global warming

When Earth Day observances first began in 1970, Cleveland had recently doused a pollutant-fueled fire on a section of the Cuyahoga River. Cities were often shrouded in thick blankets of smog. And large portions of Lake Erie were so fouled by industrial, farm, and sewage runoff that sections of the 241-mile-long lake were pronounced dead.

As an environmental issue, global warming was far down the list of concerns.

At the time, a small number of climate scientists noted a general cooling trend in Earth's climate. They even suggested Earth might be about to begin a millenniums-long slide into a new ice age.

It was a decidedly minority view, however. And it quickly faded as global average temperatures rose and as the burning of fossil fuels and deforestation pushed concentrations of greenhouse gases in Earth's atmosphere relentlessly higher.

Since that first Earth Day, the air over major cities is cleaner. Lake Erie is healthier. So is the Cuyahoga River, which groups in Cleveland would like to turn into a centerpiece of urban life. The improvements have come with "yes, but ..." as other environmental challenges have elbowed their way to the fore. But for the most part, tools are in place to deal with them.

So, how are we doing on global warming, now widely seen as the century's most pressing environmental issue?

For the most basic measure, let's turn to the atmosphere and the changes that human activities are imposing on the mix of gases it contains – primarily carbon dioxide (CO2).

Most climate scientists trace global warming to the relatively rapid buildup of atmospheric CO2 from the burning of fossil fuels long sequestered deep underground.

Though only 0.04 percent of all the gases in the troposphere, where weather happens, CO2 is second only to water vapor as the most abundant greenhouse gas. And where a water molecule may remain airborne for up to 10 days before returning to the surface as rain, a newly emitted molecule of CO2 can remain in the air for centuries.

"From the grossest physical indicator, we're not getting the job done as a planet," says Alden Meyer, director of the Union of Concerned Scientists office in Washington, of the steady rise in CO2 levels.

"But ... there's some good news in the fact that some countries are moving forward with domestic strategies" for curbing greenhouse-gas emissions, says Mr. Meyer, who closely tracks national and international efforts to deal with climate change. "Is it enough, fast enough? No. But are they better than business as usual? Yes."

Atmospheric CO2 concentrations have reached the highest level in at least 800,000 years, reaching 395 molecules of CO2 for every million molecules of all gases in the atmosphere – 395 parts per million – some 45 to 50 percent higher than preindustrial levels.

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