A new report from the United Nation's panel of climate scientists offers little in the way of surprises: Climate change, it says, is almost entirely man-made; it will be irreversible if nothing is done soon; and reducing greenhouse gases to zero this century may be necessary to reverse its effects. This isn't surprising. The Intergovernmental Panel on Climate Change's final volume, dubbed the "Synthesis Report," echoes the previous three reports that relied on the findings of more than 800 scientists released over the previous 13 months.

"Science has spoken," U.N. Secretary-General Ban Ki-moon declared at the launch of the report in Copenhagen. "There is no ambiguity in their message. Leaders must act. Time is not on our side."

The report makes it clear that scientists are more certain than ever before that human activity is responsible for climate change. "Human influence on the climate system is clear, and recent anthropogenic emissions of greenhouse gases are the highest in history," the report says. To keep the the global temperature rise from reaching what the panel views as a dangerous level, emissions from fossil fuels may need to drop to zero by the end of the century. The report cites increasingly frequent and severe heat waves, melting glaciers, and dramatically changing sea levels as indicators of the accelerating rate of climate change.  

Although the report by and large has a bleak outlook, it also strikes a hopeful tone, saying that educating the general public about climate change is the key to reversal.

"We have the means to limit climate change," IPCC chairman Rajendra Pachauri says in a press release issued with the report. "All we need is the will to change, which we trust will be motivated by knowledge and an understanding of the science of climate change."

We already have technology, the report points out, that could play a major role in helping to end our dependence on fossil fuels. "It is technically feasible to transition to a low-carbon economy," Youba Sokona, co-chair of one of the IPCC's working groups, says. "But what is lacking are appropriate policies and institutions. The longer we wait to take action, the more it will cost to adapt and mitigate climate change."

Secretary of State John Kerry called the report "another canary in the coal mine" and warned that politics and ideology should not get in the way of taking action. "The bottom line is that our planet is warming due to human actions, the damage is already visible, and the challenge requires ambitious, decisive, and immediate action," Kerry said in a statement. "Those who choose to ignore or dispute the science so clearly laid out in this report do so at great risk for all of us and for our kids and grandkids."