Causes of the current heat wave are complex. Drought, high pressure and sprawl all play roles.
The heat was unreal — so blistering that a windowsill thermometer overlooking Olympic Boulevard in downtown Los Angeles blew its top when the mercury hit 130 degrees. People consumed so much water that parts of the city briefly ran dry. Four people died. Dozens were hospitalized.
It was still 89 degrees at 1 a.m.
The record hot spell did not occur in 2006, but 1955, long before scientists raised the prospect of global warming and climate change.
The extreme temperatures of this year’s heat wave have been so intense that they have created a sense of fundamental change — that somehow Los Angeles is on the verge of a searing future.
But few events occur with such regularity or are so quickly forgotten as Southland heat waves, with extremes of temperature rising and falling in a regular rhythm like rolling curls of surf.
Climate experts at the Jet Propulsion Laboratory in La Cañada Flintridge and the Scripps Institution of Oceanography in La Jolla cautioned Tuesday that no single event — no matter how unusual — could be directly attributed to global warming and the effects of pollution.
There is such natural variability in temperature that even a record scorcher is just one data point in a long temperature timeline.
«To call it global warming would be overdoing it,» said climatologist Daniel R. Cayan of Scripps and the U.S. Geological Survey. «This is largely natural variability.»
But the current heat wave, which has been brewing since May, has nonetheless raised alarms. It is simmering with sustained intensity, echoing record high temperatures now wilting Europe and Asia.
«There may be some exacerbating climate change ingredient,» Cayan said. «In fact, it is almost certain.»
The current high temperatures fit with extremes that have been on an upward arc for the last century and are in line with computer projections for more records in the future.
«What we now call extreme events are becoming run-of-the-mill happenings,» said Scripps climatologist Tim Barnett.
The first six months of 2006 were the warmest in the United States since record-keeping began in 1895, according to the National Climatic Data Center. The 10 hottest years on record have all occurred since 1990, a trend that a majority of scientists say is in large part attributable to human production of greenhouse gases, which trap heat in the atmosphere.
All told, the planet has been slowly warming for a century, with Earth’s average temperature rising by 1.6 degrees. In Los Angeles, the average daytime temperature has increased 3 degrees over the last century, while nighttime temperatures have increased 7 degrees, records show.
In 1939, a high of 107 degrees broke all records. By 1955, the record high was 108 degrees; it crept to 109 degrees by 1963, and in 1990 reached 112 degrees.
Such temperature extremes arise from a cat’s cradle of causes, experts said. The current weather is affected by an extended regional drought and broader, long-term climate trends that encompass much of the Northern Hemisphere.
The effects of urban development also play a major role, as thousands of square miles of dry chaparral are transformed into highways, housing tracts and strip malls — all of which retain heat.
The immediate cause of the current heat is a lingering high-pressure system centered over the Four Corners region of the Southwest, said JPL climatologist William Patzert.
As it slowly turns clockwise at about 15 mph, that immense wheel of air also sweeps the ocean’s warm surface water against the Southern California coast, eliminating the cooling marine breeze that tempers the local climate, he said.
An extended drought in the Western states has strengthened the high-pressure system, while the jet stream, which in a normal year would help cool the West, has kept north of the Canadian border.
«This heat wave is coast to coast, border to border,» Patzert said. «It has been going on for six weeks now where temperatures have been abnormally high. Now they are off the scale.»
The patterns have come and gone in the past.
In July 1931, sweltering Angelenos bemoaned the 37th straight day of extreme high temperatures — at that point the longest stretch of hot, humid local weather in the history of the National Weather Service.
Few recalled that, a generation earlier, as temperature records shattered in July 1891, perspiring businessmen sought shelter in the cool of the Grand Opera House and worried that such searing temperatures might mar efforts to market California’s perfect climate to Easterners.
No one then would have blamed global warming — a concept that did not gain scientific currency until the 1980s.
Since then, scientific understanding has progressed in lock step with a contentious political debate.
The debate eludes resolution because of the difficulty of separating normal temperature swings from longer trends. In the effort to understand climate, certainty comes only with the hindsight of centuries.
The severity of the current heat wave, in which temperatures this month have reached 100 degrees or more for at least 10 straight days, marks the first time in 57 years that both Northern and Southern California have experienced simultaneous, extended high temperatures, California’s Undersecretary for Energy Affairs, Joe Desmond, said Tuesday.
«This is a historic heat wave,» Desmond said.
Still, Patzert said of California’s weather: «Is that a part of global warming? I don’t know.»
Some scientists, however, believe it a harbinger of more extreme summers in decades to come.
«People talk about tipping points,» said Scripps’ Barnett. «We have gone past it. There is nothing we can do to stop it now. The only question is how big a hit we are going to take.»
Whatever the ultimate scientific truth, this month’s weather has been for many Southern Californians a perceptual tipping point that brought home the possibility of global warming, just as the fury of Hurricane Katrina did for the people of New Orleans.
Inside the air-conditioned darkness of the Majestic Crest Theatre in Westwood, Max Furstenau, 18, was cleaning up after Tuesday’s 3 p.m. showing of «An Inconvenient Truth,» in which former Vice President Al Gore made the case for global warming.
Outside, the weather had finally cooled to the comfortable mid-80s. The day before had hit 110 degrees, breaking the record of 107 set in 1954.
«I know it’s happening,» Furstenau said.
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