August 2, 2010 | NCAR’s pinprick of a foothold on the planet, Boulder, experienced a cool and snowy spring, but elsewhere on Earth, the mercury has been startlingly and consistently high during the first half of 2010.
Warm temperatures have been present over most of the world, particularly eastern North America, parts of Asia, Eastern Europe, and equatorial Africa. By late June, seven countries in the Middle East and Africa had set new records for their all-time hottest temperatures.
In mid-July, NOAA reported that June 2010 was the hottest June on record (since records began in 1880) when taking the combined global land and ocean surface temperature into account. For the year to date, the combined global land and ocean surface temperature made January–June the warmest such period on record, and the second warmest (behind 2007) when looking only at average land surface temperature. In June, researchers at both NASA and Britain’s Met Office were predicting that the heat trend was likely to continue for the rest of the year, with more than a 50% chance of 2010 ending up the hottest year on record.
At NCAR, scientists are researching the rising heat in all its forms, from big changes in global climate down to localized heat waves.
The search for “missing” heat
Current observational tools cannot account for roughly half of the heat that is believed to have built up on Earth in recent years, according to NESL/CGD’s Kevin Trenberth. Based on information through 2008, Kevin published a “Perspectives” article in Science in April 2010 with co-author John Fasullo (also in CGD) warning that satellite sensors, ocean floats, and other instruments are inadequate to track this “missing” heat, which may be building up in the deep oceans or elsewhere in the climate system (news release).
In their study, Kevin and John focused on a central mystery of climate change: whereas satellite instruments indicate that greenhouse gases are continuing to trap more solar energy (heat) in the atmosphere, since 2003 scientists have been unable to determine where much of that heat is going.
They suggested that either satellite observations are incorrect or, more likely, large amounts of heat are penetrating to regions that are not adequately measured, such as the deepest parts of the oceans. Compounding the problem, Earth’s surface temperatures had largely leveled off in recent years. Yet melting glaciers and Arctic sea ice, along with rising sea levels, indicate that heat is continuing to have profound effects on the planet.
“The heat will come back to haunt us sooner or later. The reprieve we’ve had from warming temperatures in the last few years will not continue,” Kevin said, adding that perhaps 2010 is a manifestation of this. “It is critical to track the buildup of energy in our climate system so we can understand what is happening and predict our future climate.”
As greenhouse gases accumulate in the atmosphere, satellite instruments show a growing imbalance between energy entering the atmosphere from the Sun and leaving from Earth’s surface. But tracking this growing amount of heat is far more complicated than measuring temperatures at the planet’s surface. The oceans absorb about 90% of the solar energy that is trapped by greenhouse gases. Additional amounts of heat go toward melting glaciers and sea ice, as well as toward warming Earth’s land and parts of its atmosphere. Only a tiny fraction warms the air at the planet’s surface.
Satellite measurements indicate that the amount of greenhouse-trapped solar energy has risen over recent years while the increase in heat measured in the top 3,000 feet of the ocean has stalled. Although it is difficult to quantify the amount of solar energy with precision, Kevin and John estimate that, based on satellite data, the amount of energy inbalance appears to be about 1.0 watts per square meter or higher, while ocean instruments indicate an imbalance of about 0.5 watts per square meter, leaving scientists unable to account for about half the total amount of heat.
Since the Science article, scientists have put forth more recent estimates of ocean heat content changes since 1993. In a “News and Views” commentary in Nature (PDF available), Kevin points out that while the ocean has been warming at a rate consistent with radiative imbalance estimates from human-caused climate change, a slowdown in warming from 2003 to 2008 is at odds with top-of-atmosphere radiation measurements. “This discrepancy suggests that further problems may be hidden within the ocean observations and their processing,” he writes. “It also highlights the need to do better.”
Kevin and John call for additional ocean sensors, along with more systematic data analysis and new approaches to calibrating satellite instruments, to help resolve the mystery. “Global warming at its heart is driven by an imbalance of energy: more solar energy is entering the atmosphere than leaving it,” John says. “Our concern is that we aren’t able to entirely monitor or understand the imbalance. This reveals a glaring hole in our ability to observe the buildup of heat in our climate system.”
Keeping tabs on sea ice
Some of that missing heat appears to be going into observed melting of sea ice in the Arctic and ice sheets in Greenland and Antarctica, Kevin and John say.
Indeed, the July 6 update from the Boulder-based National Snow and Ice Data Center reported rapid Arctic sea ice loss through June. The average June ice extent was the lowest in the satellite data record (1979 to present), and Arctic air temperatures were higher than normal. June saw the return of the Arctic dipole anomaly, an atmospheric pressure pattern that contributed to the record sea ice loss of 2007. In July, however, weather patterns shifted, slowing the pace of ice loss.
“The atmospheric conditions in the last couple of months look very favorable for sea ice loss,” said Jen Kay (NESL/CGD) in early July. “If this pattern persists, I predict we are in store for another record low year.”
She cautioned, however, that there was still a lot of melting left to happen, as July and August are when the greatest sea ice loss occurs each year. Weather conditions, atmospheric patterns, and cloud cover over the next month will determine whether the 2010 sea ice declines to a level similar to 2007. The timing of the ice extent loss also matters, Jen points out. Melting early in the season leads to strong albedo feedbacks, as the presence of dark, open water when the Sun is at its seasonal peak causes the upper ocean to absorb more solar radiation, enhancing sea ice melt.
Hot time in the city
During the first week of July, an intense heat wave crippled the East Coast, breaking records from Washington, D.C., up to Boston. Although an individual weather event cannot be attributed to climate change, research shows that climate change can shift the odds toward events such as heat waves.
Claudia Tebaldi, a research scientist at Climate Central and regular collaborator with NESL/CGD who is visiting the division this summer, collaborated on a report released by Climate Central on July 6 analyzing projected midcentury July and August temperatures for several major American cities, under a fairly conservative warming scenario. The report found that startling changes could lie ahead, with more cities breaking 90°F and 100°F more often.
Claudia calculated the changes in hot days using National Climatic Data Center daily data for the current climatology and model output data compiled by the World Climate Research Programme for the projected changes.
She stresses that it’s not possible to say with certainty that the recent East Coast heat wave would not have happened in the absence of global warming. “We can say, however, that global warming is already loading the dice and making this kind of event more likely,” she says. “We’re trying to raise awareness of the fact that these uncomfortable and costly episodes are deemed to become much more frequent in our not-so-far future because of global warming. It's in our power to try to mitigate them and their effects.
Preparing for a warmer world
A multidivisional team of NCAR researchers hopes to do just that—help society prepare for and mitigate heat waves, which are the leading cause of weather-related deaths around the world. The team, led by Olga Wilhelmi (ISP/RAL), has launched SIMMER (System for Integrated Modeling of Metropolitan Extreme Heat Risk). Their goal is to develop both a methodology for assessing urban vulnerability to heat waves and a system for building capacity in the public health sector to mitigate and adapt to heat waves.
The project, which kicked off in late July, evolved from a paper that Olga and Mary Hayden (ISP/RAL) published in March 2010 in Environmental Research Letters that details a new research framework for reducing urban vulnerability to extreme heat. “The framework provides a model of how applied research should be done from macro level climate modeling down to local level decision making,” Mary says.
SIMMER will apply this conceptual framework to Houston and Toronto, two very different cities. The researchers will use CCSM and WRF to model changes in extreme heat events across the United States and Canada, improve methods for identifying urban heat vulnerability, and produce new local and regional datasets for the broader research community.
NCAR is leading the project, which will run for the next three years. In addition to Olga and Mary, Andy Monaghan (RAL) is a co–principal investigator. Partners include government and public health agencies in Houston and Toronto, a handful of universities in both countries, and NASA.