As if the public debate about global warming wasn't complex enough, a new field in climate research is coming of age, grabbing media attention and spawning seemingly contradictory headlines.

The work, called attribution research, doesn't challenge the scientific consensus that climate change is happening. Instead, it strives to understand the regional effects of global warming by determining whether increased greenhouse gas levels did—or didn't—cause a particular weather event. But the findings can be confusing. For instance, scientists say that climate change made Hurricane Sandy worse, but that it had nothing to do with historic floods in Thailand. Warming probably didn't cause this year's severe winter storm, Nemo, but it may have supercharged it. It caused some droughts, but not others.

Adding to the confusion is a trend that worries some experts: People on both sides of the climate divide tend to promote only those attribution studies that support their beliefs. Whether a conscious decision or not, this selective use of research is further polarizing the national conversation on climate change, experts told InsideClimate News.

In the latest show of force by opponents of the Keystone XL pipeline, a group of 150 major Democratic donors sent a letter Friday to President Obama, urging him to reject the controversial application from TransCanada for permission to send more than 800,000 barrels of tar sands oil a day from Alberta to the Texas Gulf Coast.

The signatories comprise business leaders, philanthropists and celebrities—including clean energy entrepreneurs Vinod Khosla, Jigar Shah and Steve Kirsch, long-time Obama bundler Wendy Abrams and actress Blythe Danner.

For more than a month, residents of Mayflower, Ark. have been told not to worry about lingering fumes from a March 29 oil spill that shut down a neighborhood and forced the evacuation of 22 homes.

"Overall, air emissions in the community continue to be below levels likely to cause health effects for the general population," Arkansas regulators wrote on a state-operated website that tracks Mayflower's air monitoring data.

Despite these reassurances, residents have suffered headaches, nausea and vomiting—classic symptoms of short-term exposure to the chemicals found in crude oil.

"Figuring out how to protect people after a disaster like this is very hard," said Aaron Bernstein, a public health expert and associate director of Harvard's Center for Health and the Global Environment. "People living near the spill early on could definitely have gotten sick" from the concentrations present in the air.

A key piece of data related to the biggest tar sands oil spill in U.S. history has disappeared from the Environmental Protection Agency's website, adding to confusion about the size of the spill and possibly reducing the fine that the company responsible for the accident would be required to pay.

The July 2010 accident on an Enbridge Inc. pipeline dumped thousands of barrels of Canadian dilbit into the Kalamazoo River and surrounding wetlands. But almost three years and two federal investigations later, one of the most important questions about the spill remains unanswered: Exactly how much oil spilled from the pipeline?

The same question is being asked about a more recent dilbit spill—a March 29 accident on ExxonMobil's Pegasus pipeline in Mayflower, Ark. Estimates for that spill, which is still being cleaned up, have risen from 80,000 gallons to more than 200,000 gallons.

Determining the size of an oil spill is important, because every barrel of oil that reaches a navigable waterway triggers a statutory fine of $1,100 per barrel under the Clean Water Act. The fine rises to $4,300 per barrel if a company is proven to have acted with gross negligence.