When the increases in the atmospheric concentrations of CO2 are combined with those of other greenhouse gases, such as methane and nitrous oxide, the overall concentrations are the highest in at least 800,000 years. The current rates of increase in these gases "are unprecedented in the last 22,000 years," the draft states.
The recent pause in the rate of increase in warming has left researchers scurrying to unravel the mystery. The upper layers of the earth’s oceans are a lead suspect for absorbing more heat that otherwise would remain in the atmosphere.
Still, each of the past three decades have been warmer than all of the previous decades since the mid-1850s, when regular record-keeping began, the draft says. The first decade of this century topped them all. The past 30 years have "very likely" been the warmest in the past 800 years, and "likely" the warmest three decades in the past 1,400 years.
The IPCC reports represent scientific time capsules. They provide an overview of the state of the science, but the research is more than a year old by the time the volumes hit the streets. Because these reports are influential, this lag has led to criticisms that their conclusions can be too conservative.
For instance, when the first working group's volume was released in 2007, researchers criticized it for failing to include in its sea-level projections the contributions from melting ice sheets in West Antarctica and Greenland. The working group didn't include those because they weren't well understood. Yet research published after the deadline for that report indicated that the melting appeared to be increasing.
This time around, Greenland and Antarctica have reappeared in the mix. The draft's authors suggest that under the worst-case emissions scenario the modelers considered, global sea levels could rise by up to 1 meter by the end of the century, about two centimeters higher than the top of the range offered in 2007.