First, understand this: The world is getting warmer. And scientists say we’re responsible. Global climate change is real.

In the United States, 2006 was the warmest year on record. Many think it will be remembered as the seminal year for the country’s understanding and acceptance of the scientific consensus on climate change.

Al Gore’s Oscar-winning documentary “An Inconvenient Truth” was released. A heat wave killed 140 people across the state. And the worst wildfire season since 1960 raged across the United States.

But 2007 has been equally important. In February, the United Nations’ Intergovernmental Panel on Climate Change released a report that effectively cemented the scientific certainty that humans are warming the planet.

So what does this mean for us in San Diego?

Scientists say San Diego can expect an average 4-degree Fahrenheit temperature increase by 2100 — tempered because of its proximity to the ocean. Further inland near El Centro, the increase may be as high as 8 degrees Fahrenheit.

The effects of a warmer planet are expected to wreak havoc on San Diego’s water supply. A warmer world means more precipitation falls as rain. The snow that does fall is expected to melt faster. Scientists say the planet’s slowly warming temperature is already causing snow packs to melt about a week earlier than in the 1950s. By 2050, they say, those melts will happen three weeks earlier. By 2100, they project it will be a full month.

The trend could have serious implications in San Diego — an arid region that gets more than 80 percent of its water supply from rivers fed by melting snow packs in the Sierras and the Rockies.

And as the world warms, the belt of deserts that encircles the planet could extend north, scientists say. That belt stretches from the Australian outback to the Sahara to the Sonoran desert in Mexico. San Diego sits on its northern fringe.

While scientists agree that humans are warming the planet, many questions are still left to be resolved. Will hurricanes grow stronger? How will specific regions be impacted?

Many researchers in San Diego are working to answer those questions. Scientists at Scripps Institution of Oceanography at the University of California, San Diego are at the forefront of climate research. Four Scripps scientists contributed to the landmark IPCC report released in February.

Researchers at Scripps and other institutions are working to answer questions about the implications for Pacific Ocean hurricanes, which occasionally form off the coast of Baja California. In San Diego, will global warming increase or decrease the small risk of a hurricane strike? The city was hit by a weak hurricane in 1848.

Such an event now could cause more than a billion dollars in damage, scientists say.

Another of climate change’s uncertainties: Sea-level rise. Polar ice cap melt and the expansion of the ocean’s volume — warmer water occupies more space — is expected to make oceans rise. San Diego has seen a 7-inch rise since 1900.

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