The list of global warming news is not optimistic: The world is spewing greenhouse gases at a faster rate. Summer Arctic sea ice is at record lows. The ice sheets in Greenland and West Antarctica are melting quicker than expected.
Is he the doomsayer global warming skeptics have called him?
Mann laughs. This Penn State University professor - and many other climate scientists - are sunny optimists. Hope blooms in the hottest of greenhouses.
Climate scientists say mankind is on the path for soaring temperatures that will melt polar ice sheets, raise seas to dangerous levels, and trigger mass extinctions. But they say the most catastrophic of consequences can and will be avoided.
They have hope. So should you, Mann said.
"Sometimes we fear that we are delivering too morose a message and not conveying enough that there is reason for optimism," Mann said.
Mann is not alone in laughing, even though the news he delivers could make people cry.
"It's hard at times," said University of Victoria climate scientist Andrew Weaver. "You can't give up hope because what else is there in life if you give up hope? When you give up hope, that's quitting and scientists don't like to quit."
That optimism is based on science and faith.
The science, Mann said, is because climate researchers are sure of one thing that the public is not: The numbers show that there is still time to avert the worst.
NASA's James Hansen, who forecasts some of the bleakest outlooks on global warming, said in an e-mail: "I am always surprised when people get depressed rather than energized to do something. It's not too late to stabilize climate."
"I am not about to give up," Hansen wrote. He has hope, he says, because he has grandchildren.
The scientists say the public now understands how bad the problem is. So these researchers have faith that society will rally in time.
Bob Corell, an American Meteorological Society climate scientist, is hopeful because even industry is pushing for change - and will make money in the deal.
Mann points to an international agreement 20 years ago this month that stopped the worsening global problem of ozone depletion. The same can be done for global warming, he said.
If the world spews greenhouse gases at its current ever rising rate, expect a 7-degree Fahrenheit (4-degree Celsius) rise by the end of the century. If those gases are curbed, then warming can be kept to about 1 degree F (0.6-degree C), an international panel of experts said earlier this year.
How about Al Gore? Does he lose hope?
"No, because we can't afford to," said the former vice president, who has helped bring global warming to center stage. "It's a genuine planetary emergency."
Optimism in the face of gloomy data is not surprising, said psychologist David Myers of Hope College in Holland, Michigan.
"Human beings are remarkably resilient," said Myers, who studies the psychology of happiness. "To do what climate researchers are doing takes enough optimism to sustain their hope and enough realism to create their concern."
Stanford University climate scientist Stephen Schneider has battled cancer and it has colored his outlook. He said the key is not to get overwhelmed by the belief that something is too tough. Sure a 2-degree F (1.2-degree C) rise in temperatures is bad. But 4 or 5 degrees F (2.4 or 3 degrees C) would be even worse.
Schneider's wife, Stanford University biologist Terry Root, recalled how in 2002 she was sitting at the hospital as Schneider slept after cancer treatment. The oncology nurse came in, chatted and asked her what she did for a living.
Root said she studied how animals are being hurt by global warming. "That is such a depressing job," replied the nurse who daily deals with cancer patients.
Then they both laughed.