When Dr. Stanley Fineman started as an allergist in Atlanta, he told patients they should start taking their medications and prepare for the drippy, sneezy onslaught of pollen season around St. Patrick’s Day. That was about 40 years ago.

Now he tells them to start around Valentine’s Day.

Across the United States and Canada, pollen season is starting 20 days earlier and pollen loads are 21% higher since 1990 and a huge chunk of that is because of global warming, a study found in Monday’s journal the Proceedings of the National Academies of Sciences.

While other studies have shown North America’s allergy season getting longer and worse, this is the most comprehensive data with 60 reporting stations and the first to make the required and detailed calculations that could attribute what’s happening to human-caused climate change, experts said.

“This is a crystal clear example that climate change is here and it’s in every breath we take,” said lead author Bill Anderegg, a biologist and climate scientist at the University of Utah, who also has “really bad allergies.”

The warmer the Earth gets, the earlier spring starts for plants and animals, especially those that release pollen. Add to that the fact that trees and plants produce more pollen when they get carbon dioxide, the study said.

“This is clearly warming temperatures and more carbon dioxide putting more pollen in the air,” Anderegg said. Trees are spewing the allergy-causing particles earlier than grasses, he said, but scientists aren’t sure why that’s the case. Just look at cherry blossoms opening several days earlier in Japan and Washington, he said.

Texas is where some of the biggest changes are happening, Anderegg said.

The South and southern Midwest are getting pollen season about 1.3 days earlier each year, while it’s coming about 1.1 days earlier in the West, he said. The northern Midwest is getting allergy season about 0.65 days earlier per year, and it’s coming 0.33 days earlier a year in the Southeast.

Anderegg said his team factored that in that parks and plants in cities were getting greener. They did standard detailed calculations that scientists have developed to see if changes in nature can be attributed to the increase of heat-trapping gases from the burning of coal, oil and natural gas. They compared what’s happening now to computer simulations of an Earth without human-caused warming and rising carbon dioxide in the air.

Since 1990, about half of the earlier pollen season can be attributed to climate change — mostly from the warmer temperatures — but also from the plant-feeding carbon dioxide, Anderegg said.