Summer brings enjoyment of warm weather, long days, a refreshing swim and barbecues. But it also brings natural hazards. Temperatures in many parts of California are near or exceed 100 degrees. Many cities are also experiencing record high nighttime temperatures, and so the evenings bring little relief from the sweltering daytime heat.

These hot, dry conditions mean wildfires. More than 4,200 fires have already burned parts of California, 20 percent more than the average over the past five years. In addition to threatening homes, fires create air quality problems, in some cases far away from the site of the fire.

But while many Californians are exposed to these conditions, some are far more vulnerable than others. Children and the elderly, for example, are more susceptible to heat-related illnesses and other hazards. Or when a fire or flood occurs, those with low incomes may not have the resources to evacuate. They are also often underinsured and unable to rebuild after the disaster strikes.

A series of reports recently released by the state point to a future climate in California that is marked by increases in extreme heat, wildfires, coastal flooding and erosion. Changes are also likely to occur in air quality, water availability and the spread of infectious diseases. But understanding the physical impacts of climate change is only half the story.

Climate risk is a function of our exposure and our vulnerability. Social factors, like age, race and income, are not tied to a specific hazard but will greatly determine the human toll of the hazard and the specific needs for preparedness, response and recovery.

To compare overall social vulnerability – the susceptibility of a given group of people to harm from a hazard – to climate change across the state, the Pacific Institute developed a new "climate vulnerability index" that combines data from 19 different social and economic factors – such as air conditioner ownership, percentage of tree cover, workers in outdoor occupations and more – for each of the state's 7,000 census tracts.

Our analysis shows that 12.4 million Californians currently live in census tracts with high social vulnerability to climate impacts – and a disproportionate number are in the Los Angeles area. Approximately 27 percent of the state's population lives in Los Angeles County – yet more than 40 percent of those with high social vulnerability, or about 5 million people, are in L.A. County. And in some rural counties, the number of people living in areas with high social vulnerability is not large, but that number represents a large fraction of the total population in the county. In Merced County, for example, 70 percent of the population lives in areas with high social vulnerability.

The greatest risks from climate change are found in areas with both high exposure to a climate impact and high vulnerability. By midcentury, Sacramento is expected to experience up to 31 extreme heat days each summer with temperatures in excess of 102 degrees – conditions that now occur on only the eight hottest days of the year. But while warmer temperatures will affect all Californians, they will be especially problematic for the elderly and others with heightened vulnerabilities, many of whom are concentrated in Southern California and in the San Joaquin Valley.

Wildfires will become increasingly common across Southern California. By the end of the century, the likelihood of a fire over large swaths of Southern California is projected to be 33 percent or higher. L.A. County has more than 3.7 million people with high social vulnerability living in these areas; large numbers are also found in San Diego, San Bernardino and Orange counties.

In coastal areas, rising sea level can result in flooding. Sea level along the California coast is projected to rise from 1 to 1.4 meters by the end of the century, with coastal flooding largely centered on the San Francisco Bay Area and in the Los Angeles region. While social vulnerability in areas at risk of flooding is typically low in Southern California, it is much higher in the Bay Area.

There is mounting evidence that climate impacts are already affecting our daily lives. Communities must begin to plan and prepare for these impacts. But we must move beyond strictly focusing on the impacts. We must also consider social vulnerability to these impacts.

Understanding vulnerability factors – and the populations that exhibit these factors – are critical for crafting effective climate change adaptation policies and response strategies. A vulnerability analysis can highlight geographic areas where targeted assistance is needed and guide discussions about how to distribute climate adaptation funds. It could also help identify adaptation strategies that would be most effective in a particular area. For example, policies that encourage residents to weatherize their homes may not be effective among those that cannot afford to make the improvements.

To address the vulnerabilities identified in the study, communities need to begin developing and implementing adaptation plans. But most importantly, the local planning processes must involve the communities most vulnerable to harm to ensure that these plans are just, appropriate and actionable.

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