WASHINGTON — In its most comprehensive study so far, the nation’s leading scientific body declared on Wednesday that climate change is a reality and is driven mostly by human activity, chiefly the burning of fossil fuels and deforestation.

The group, the National Research Council, an arm of the National Academy of Sciences, issued three reports describing the case for a harmful human influence on the global climate as overwhelming and arguing for strong immediate action to limit emissions of climate-altering gases in the United States and around the world — including the creation of a carbon pricing system.

Congress requested the reports in 2008. This is the first time the academy has issued specific recommendations on how to mitigate or adapt to climate change.

One of the reports, “Limiting the Magnitude of Future Climate Change,” urges the United States to set a greenhouse gas emissions “budget” that restricts overall emissions and provides a measurable goal for policy makers and for industry. It does not recommend a specific target but says the range put forward by the Obama administration and Congress is a “reasonable goal.”