If greenhouse-gas emissions continue to grow unchecked, the maximum temperatures, rainfall, and other aspects of climate that humans have experienced during the past 150 years will become the new minimum globally by 2047 (give or take 14 years), according to a new study.
That year applies to global averages. For specific locations, this shift could come as early as 2020, the results show.
The shift appears soonest in the tropics, where some 5 billion people live – many of them among the world's poorest – and where the planet hosts the highest levels of biodiversity, according to the study's projections. Based on one metric (ocean acidification), today's norms already became the new minimum in 2008.
The timing for the consistently altered climate depends on how quickly greenhouse-gas emissions from human industrial activity build in the atmosphere, the results show.
Using emissions scenarios in reports by the UN-sponsored Intergovernmental Panel on Climate Change (IPCC), the team estimates that relatively aggressive efforts to curb emissions could delay the switch by up to 30 years. But even with strong curbs on emissions, the shifts will still take place.
These changes "will result in environments like we have never seen before," says Camilo Mora, a biogeographer at the University of Hawaii at Manoa and the lead author of the study, which appears in Thursday's issue of the journal Nature.
On one level, the study reaffirms a point other researchers have made in the past: that by mid-century the fingerprints of global warming will become more easy to distinguish from the ups and downs of natural climate swings.
But this study adds fresh elements to the story, researchers say.
One is a more-specific sense of timing. Dr. Mora's team delivers projections for 26 major cities. Under business as usual for emissions, the region around Manokwari, a provincial capital in eastern Indonesia, would see the shift in climate variability in 2020, give or take five years. More-aggressive emissions controls would delay that shift by about five years, the researchers estimate.
Anchorage, Alaska, on the other hand, would see the latest onset of changes as early as 2071 with business-as-usual emissions and 2095 with tougher global emissions controls.
Another is its focus on trends in climate variability across broad regions – particularly in the tropics – rather than on a single global average.
This "is desperately needed," says Frank Lowenstein, who focuses on adaptation to global warming for the Nature Conservancy, an environmental group based in Arlington, Va. Organisms in an ecosystem or culverts that civil engineers design to divert rain water are set up to endure the extremes, not the average, he explains. Changes to climate extremes can put additional stress on organisms and well as on societies.
Mora's team notes that widespread adverse effects from the shift are likely to appear earliest in the tropics because marine and terrestrial plants and animals there have adapted to a climate with relatively with small shifts in extremes.
The team based its study on 39 climate models the IPCC used for its latest climate-assessment reports and used climate records from 1860 to 2005 to determine historical ranges of natural variability.
While the researchers express the changes in terms of temperatures near the surface, they also examined precipitation, evaporation, the movement of water through plants, heat transferred from Earth's surface to the atmosphere, as well as ocean acidity. The researchers found that with the exception of ocean acidification, whose variability already falls outside of historic bounds, these other actors in the climate system will shift to new minimums that exceed past maximums somewhat later than near-surface temperatures.
The study Mora's team has produced "goes a long way toward pinning down the time line," notes Kevin Trenberth, a researcher at the National Center for Atmospheric Research in Boulder, Colo., who was not a member of Mora's team but whose research focuses on climate change and climate variability.
But the conclusions about the tropics may be suspect, he says in an e-mail. The main source of variability in the tropics is the swing between El Niño and La Niña. These involve shifts in warm sea-surface temperatures and air-pressure patterns in the tropical Pacific that seesaw from east to west and back every two to seven years. The team showed no evidence of evaluating how well the models replicate the frequency and intensity of these important climate swings.
Some models overstate the intensity of the climatological siblings, known collectively as the El Niño-Southern Oscillation (ENSO), while others produce no ENSO at all.
And since water resources represent a critical element for ecosystems and societies, "one would like to know a lot more about monsoons" and other seasonal changes in precipitation in the tropics.
For his part, Mr. Lowenstein says he suspects the team's results are too conservative. It bases its timing estimates on the year after which all succeeding years display the new climate regime. But, says Lowenstein, a looming shift in climate regimes can have a significant effect on people and ecosystems even if the new range of variability occurs in five years out of 11, instead of 11 out of 11.
Still, he and other researchers say the study provides useful insights into the projected patterns, timing, and effects of a new climate regime and the importance of putting a serious brake on emissions while helping the countries earliest hit and least able to cope adapt to the coming changes.