The amount of heat-trapping carbon dioxide in the atmosphere reached a record 390.9 parts per million (ppm) in 2011, according to a report released Tuesday by the UN's World Meteorological Organization (WMO). That's a 40 percent increase over levels in 1750, before humans began burning fossil fuels in earnest.

Although CO2 is still the most significant long-lived greenhouse gas, levels of other heat-trapping gases have also climbed to record levels, according to the report. Methane, for example hit 1813 parts per billion (ppb) in 2011, and nitrous oxide rose to 324.2 ppb. All told, the amount of excess heat prevented from escaping into outer space was 30 percent higher in 2011 than it was as recently as 1990.

Atmospheric CO2 at Mauna Loa Observatory. Credit: NOAA.

These are sobering numbers, not because they come as any sort of surprise, but rather because they don't. Scientists have known about the heat-trapping properties of CO2 since the mid-1800s. They've been documenting the steady rise of CO2 pumped largely out of smokestacks and exhaust pipes since the 1950s.

About half of the excess CO2 going into the atmosphere so far has been absorbed by plants and the oceans, but, said WMO Secretary-General Michel Jarraud in a press release, " . . . this will not necessarily continue in the future" as these natural "sinks" for CO2 reach their capacity.

The CO2 that remains in the atmosphere, meanwhile, takes centuries to dissipate, which is why the numbers continue to climb. As a result of all the extra CO2 pumped into the air, worldwide average temperatures have already risen by 1.8°F since 1900.

Yet despite all of this knowledge, the world has largely failed to act on reducing emissions. The best they could do at a UN-sponsored climate meeting in Copenhagen in 2009 was to agree to a non-binding target of limiting the world's greenhouse-gas-triggered temperature increase to no more than 2°C (3.6°F) above preindustrial levels to limit the potential damage. Just a year later, it was already clear that they wouldn't come close to making it.

Frustrated with this global inaction, the World Bank released a report on Sunday saying that without significant emissions reductions, the world's average temperature could climb by 4°C (7.2°F) by as early as 2060. The report highlighted the dire consequences for human health and safety — including dangerous sea level rise, heat waves, and other extreme weather events.

But the potential disruption to people and property are so enormous that the report is, if not a wake-up call, at least another attempt to rouse world leaders after too many false starts and stops.

It calls not just for a reduction in CO2 emissions, but also for an aggressive program to reduce other drivers of global warming that might be easier to control including not just short-lived but powerful greenhouse gases like methane, but also heat-absorbers such as black carbon — essentially, soot.

Unlike CO2, which stays in the atmosphere for a century or more, black carbon and other so-called "short-lived climate forcers" act on timescales of weeks to a few years, meaning that reducing them would yield much faster benefits.

The World Bank report also calls attention to the fact that poor people and poor nations are at the greatest risk from the dangers posed by rising greenhouse-gas levels and the changes in climate that are likely to result.