Tom Karl, director of the National Climatic Data Center, noted that 11.5 degrees F could be optimistic, explaining that "recent emissions seen are higher than the highest scenario".

For grain crops, which are a huge part of the U.S. farm economy, faster growth that comes with warmer temperatures means less time for the seeds to grow and mature, Kenworthy noted.

"Even moderate increases in temperature will decrease yields of corn, wheat, sorghum, bean, rice, cotton and peanut crops," the report predicts.

"Further, as temperatures continue to rise and drought periods increase, crops will be more frequently exposed to temperature thresholds at which pollination and grain-set processes begin to fail and quality of vegetable crops decreases."

Mr Kenworthy pointed out that significant yield reductions due to climate change have already been seen: global wheat, maize and barley production in 2002 would have been 2-3pc higher if not for the long-term yield-limiting effects of global warming trends. The combined 2002 losses in productivity worldwide cost farmers more than $4.8 billion at 2002 prices.

Weeds, diseases and insect pests benefit from warming, and weeds also benefit from a higher carbon dioxide concentration, which increases stress on crop plants and requires more attention to pest and weed control.

Livestock impact

Bill Hohenstein, director of the US Department of Agriculture's Global Change Research Program Office, said this report also offers a detailed look at animal agriculture.

Animals are particularly affected by extreme heat stress, which lowers their productivity. Increased heat, disease and weather extremes are likely to reduce productivity.

"For example, an analysis projected that a warming in the range of 9-11 degrees F (in the higher emissions scenarios) would cause a 10pc decline in livestock yields in cow/calf and dairy operations in Appalachia, the Southeast (including the Mississippi Delta) and the southern Plains regions, while a warming of 2.7 degrees F would cause less than a 1pc decline," the report notes.

Dramatic changes are already underway in western rangelands, where higher levels of carbon are reducing forage quality and promoting the spread of shrubs and invasive annuals such as cheat grass.

Reduced forage quality means more acreage is needed to provide animals with the same nutritional value, resulting in an overall decline in livestock productivity.

"Reductions in forage quality could have pronounced detrimental effects on animal growth, reproduction and survival and could render livestock production unsustainable unless animal diets are supplemented with protein, adding more costs to production," the report says.

"On shortgrass prairie, for example, a carbon dioxide enrichment experiment reduced the protein concentration of autumn forage below critical maintenance levels for livestock in three out of four years and reduced the digestibility of forage by 14pc in midsummer and by 10pc in autumn," it adds.

Because of heat, milk production declines in dairy operations, the number of days it takes for animals to reach their target weight grows longer in meat operations, the conception rate in cattle falls and swine growth rates decline. "As a result, swine, beef and milk production are all projected to decline in a warmer world," the report says.

Projected increases in air temperatures will negatively affect confined animal operations (dairy, beef and swine) located in the Central US, increasing production costs as a result of reductions in performance associated with lower feed intake and increased requirements for energy to maintain healthy livestock.

* Read the full agriculture portion of the report here.