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 science groups, which represent millions of scientists, sent the letter Tuesday, saying the severity of climate change is increasing and will get worse 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.

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.