In July 2006, a monthlong triple-digit heat wave scorched California, killing more than 25,000 cattle and reducing dairy production in the region. Land O'Lakes Creameries, which normally produces six million liters of milk daily, was short 1.5 million liters per day. All told, experts estimate that the high temperatures caused $1 billion worth of dairy shortfalls.

Extreme weather events and higher average temperatures are predicted to increase with global warming, and that's bad news for livestock producers in the U.S. and abroad. Warming will reduce grass, brush and other forage available in many areas, and it will also directly influence cows' physiology. Dairy production is optimal at cooler temperatures between 20 to 22 degrees Celsius, explains Terry Mader of the University of Nebraska–Lincoln. "For every degree above that," he says, "we'll have a decline of approximately 2 percent productivity."

Mader is one of many scientists studying the potential impacts of climate change on livestock, and how producers can mitigate them. He says that although climate models are not conclusive about whether the U.S. Midwest's average temperature will go up 2 degrees C or 5 degrees C over the next century, the physics of cows is pretty straightforward. "You have heat generated from metabolism and digestion, and then they have to cope with the environmental component," he explains, "How do they offset increased heat? They eat less." The decline in feeding results in a decline in output, whether that's meat, milk or fur. They also tend to have lower rates of conception during warmer months. "That's just physiology," Mader says.

In an upcoming issue of the journal Climatic Change, Mader and his colleagues combined his cow model with climate models to estimate milk production along three north-south transects in the Midwest under scenarios doubling and tripling climate-warming carbon dioxide. The researchers concluded that summer milk production in the U. S. would decrease by 16 to 30 percent, about double the normal summer decline. A drop in the supply could lead to an increase in the price of milk, and suppliers in some areas would also need to use electric fans to keep facilities cool for animals.

Other researchers have been looking to tap into the genetic diversity of livestock in tropical countries to create heat-tolerant breeds that can also pump out lots of milk. The problem is that a Holstein in the U.S. can produce up to 8,000 liters of milk annually, compared with lowland Brazilian breeds that are tick-resistant and heat-tolerant but are only producing just over a thousand liters of milk per annum.

Curtis Van Tassell at the U.S. Department of Agriculture's Bovine Functional Genomics Laboratory in Beltsville, Md., has been working with San Diego–based Illumina, Inc., to build gene chips that can help researchers quickly assemble cow pedigrees and analyze milk production in relation to environmental factors. In an article published this week in PLoS One, Van Tassell collaborated with Australian researchers studying 62,343 Holsteins in their country to identify genetic markers that indicate how sensitive the cow's dairy production is to temperature and to the amount of forage they receive. "In Australia," Van Tassell explains, "the sensitivity to climate change is higher than in the U.S., particularly because of the drought conditions they have seen in the last few years."

By picking a breeding bull with genes that exhibit low temperature sensitivity, Van Tassell and his co-authors estimate they can keep milk production nearly constant, about 18 kilograms per day, as the temperature effectively increases from about 60 degrees F to 90 degrees. By contrast, selecting a more temperature-sensitive bull would result in a 20 percent decrease in milk production over that same range.

Whereas the National Milk Producers Federation (NMPF) may benefit from all this research, they are not particularly keen about the government capping the industry's greenhouse gas emissions. In a statement regarding the House climate bill passed in June, the organization wrote, "NMPF supported the concept of cap and trade as long as agriculture was not a capped industry."

On the other hand, the industry has vowed to voluntarily reduce its carbon footprint by 25 percent by 2020. And in the dairy industry's quest for economic efficiency in the past 60 years, it has already reduced its carbon footprint by 41 percent. According to a study published in the June issue of the Journal of Animal Science, today's dairy producers use only 10 percent of the land, 23 percent of the feedstock and 35 percent of the water that was used to produce the same amount of milk in 1944.

But the big question is what sort of changes the industry will make in the next century. Mader says some researchers in Brazil are so concerned about climate change, they've suggested the country set its sights on goat milk. "That's a far-fetched concept!" he chuckles. "The industries will change, but we have animals in our cattle population that we can still select from."