Since 1951, Earth's climate has warmed by about 0.6 degrees Celsius, and researchers assessing the state of climate science for the Intergovernmental Panel on Climate Change (IPCC) are 95 percent certain that more than half of the warming is due to human emissions of greenhouse gases.
The statement is one conclusion in the final draft of a summary the IPCC is preparing for world policymakers on the state of the climate and climate science as part of its fifth assessment report on global warming.
The IPCC will release the full four-volume set and related summaries beginning at the end of September and into next year. The other volumes deal with current and projected effects of climate change.
The summary on climate and climate science will get word-by-word scrutiny at a meeting in Stockholm Sept. 23-26 as politicians and scientists arm-wrestle over the final text.
In the meantime, it gives clear insights into what scientists see happening to the planet's climate as human industrial activities, as well as land-use changes, pump increasing amounts of carbon dioxide and other greenhouse gases into the air.
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.
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.
The IPCC authors also have modified their entries on tropical cyclones between this final draft, and an earlier draft leaked in December. The earlier draft indicated that the number of tropical storms, hurricanes, and typhoons globally are likely to hold steady or decrease, while the frequency of the most-intense storms are more likely than not expected to increase. The final draft reduced that to a chart entry indicating that increases in intense tropical-cyclone activity are more likely than not by century's end.
But that perspective may be changing.
A study published by the Proceedings of the National Academy of Sciences in early July found that the number of tropical cyclones is likely to increase globally by the end of the century, in addition to intensifying.
The study, which used the output from upgraded models used for this new round of IPCC reports, was conducted by Kerry Emanuel, a tropical climate specialist at the Massachusetts Institute of Technology in Cambridge, Mass.
In addition, the IPCC draft acknowledges that its projections could get thrown off by events that are difficult, if not impossible, to predict so far in advance. Volcanic eruptions represent one kind of natural event whose lofting of sulfur dioxide high into the stratosphere can create tiny particles that can have a temporary cooling effect. Solar activity is another wild card.
This has prompted some researchers to explore the potential impact of these "forcings," especially solar variability, on global warming's future.
Solar physicists have been looking at trends in sun-spot behavior and characteristics over the past decade and have raised the possibility that when the current sun-spot cycle peaks in the next few months, the sun could enter an unusually long period where it generates few, if any sunspots.
Some climate scientists have looked at the potential impact of such an event and concluded that it likely would delay additional global warming – but only until the sun returned to more-normal swings in sun-spot activity.