In some areas, certainty over what is happening has increased since the IPCC published its last set of reports in 2007. Certainty over the human role has increase from "very likely" to "extremely likely," a verbal shift representing 90 percent certainly in 2007 to 95 percent for this round of reports.
The draft notes that despite the relentless build-up of greenhouse gases in the atmosphere, the rate of warming over the past 15 years has been small compared with warming between 1951 and 2012, suggesting that within an overall warming that began in the first decade of the 1900s, the climate system still displays substantial variability on time scales of a decade or so.
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.