WASHINGTON (AP) — Thirty-one of the country's top science organizations are telling Congress that global warming is a real problem and something needs to be done about it.

The groups, which represent millions of scientists, sent the letter Tuesday, saying the severity of climate change is increasing and will worsen faster in coming decades.

Eighteen groups sent a similar letter in 2009. But Rush Holt, chief executive officer of the American Association for the Advancement of Science, said the climate problem has increased and scientists are even more confident about the harm.

"There is strong evidence that ongoing climate change is having broad negative impacts on society, including the global economy, natural resources and human health," the letter states. It mentions extreme weather, water shortages, heat waves, wildfires, sea level rise, and disruption of ecosystems in the United States.

Those signing the letter include the world's largest scientific society, American Chemical Society, and groups that represent meteorologists, public health experts, biologists, Earth scientists, oceanographers, geologists, crop researchers, bug, fish and reptile experts, as well as mathematicians and statisticians.

Texas Tech climate scientist Katharine Hayhoe said the situation is like a doctor telling a patient he has cancer, with the patient saying he doesn't. The doctor then gets every oncologist in the hospital to tell the patient it is cancer, but is treatable. It's up to the patient.

"We told you everything we could," said Hayhoe. "We are like Pontius Pilate, metaphorically washing our hands."