The first is the most obvious: with the Democrats having secured control in both Houses of Congress, the energy industry expects it will soon face an outbreak of climate-change regulations. Exxon wants to be in the room to negotiate the effect of those regulations; it wants to be taken seriously on this issue and admitting the accuracy of the science is a necessary first step.
A second reason for skepticism is more oblique: in the world's most litigious country, there is always the risk that someone will sue over the health or environmental damages caused by corporate actions. The tobacco companies have already faced such court action - and penalties - and may yet face more. But one of big tobacco's current lines of defence is that its customers know that smoking is dangerous and they do it anyway. Just by sticking a ciggy between their lips, people sign a virtual waiver, allowing Philip Morris, et al, to argue that its not their fault that people do dangerous things.
Now, Exxon can make the same kind of argument. If the company had continued to claim that fossil fuels were blameless in changing the world's climate (even though they have ample evidence to the contrary), litigants ultimately might have been able to nail Exxon for lying about the safety of its products. By admitting that oil is implicated in a potential global climate catastrophe, Exxon externalizes the responsibility for its use.
At the same time as acknowledging  human responsibility for climate change, Exxon also said it had stopped funding the Competitive Enterprise Institute (CEI ) and four or five other think tanks that have been spreading misinformation about climate science. This is an apparent answer to a request from the Royal Society  that Exxon stop funding this sleazy PR battle to confuse the public about global warming.
But ExxonSecrets  has tracked Exxon dollars to many more other think tanks, and there is no evidence that Exxon intends to completely stop its climate confusion campaign. Even where CEI is concerned, Exxon may have withdrawn support only because the institute has become a public embarrassment. CEI is no longer useful as a climate change denier because everyone knows it's an astroturf group. It has become too famous for taking industry money and therefore can no longer claim a mantle of impartiality.
Exxon may actually have turned over a new leaf. It's enormously encouraging to see the company edging toward a reasonable position. But we will await with interest a gesture more convincing than this first tentative step.