Meeting recently in South Africa, representatives from 194 countries agreed to the Durban Platform, the latest effort to put the world on a path to cut greenhouse gas emissions that scientists say are driving climate change.

One crucial question is whether the Durban conference was even addressing a real issue. For many conservatives, the answer is no. Global warming, it’s said, is a (flawed) theory, not a fact. The idea that human activity is in any way responsible for higher temperatures is false. Advocates of global warming are relying on doctored data. Indeed, global warming is a manufactured crisis being used by environmentalists to impose their left-wing agenda on America. Or so the argument goes.

Having looked into this matter a bit, I’ve settled on several judgments which are open to refinement and amendment, including these: The world is getting warmer. The warming is almost certainly caused, at least in large part, by human activity. And rising temperatures could pose a future risk, though how significant of a risk is open to interpretation.

Here’s what we do know. According to a report by the National Academy of Sciences, the average temperature of Earth’s surface increased by about 0.8 degrees Celsius during the past 100 years, with more than half occurring during the past three decades. During one recent 12-year stretch (1995-2006), 11 of those years ranked among the 12 warmest years in the instrumental record of global surface temperature. Richard A. Muller, a professor of physics who once counted himself a skeptic about global warming, re-examined the data through the auspices of the Berkeley Earth Surface Temperature Project and came to this conclusion: “Global warming is real.” And the preponderance of the scientific evidence points to human activity as the most likely cause for most of the global warming that has occurred over the last half-century. Gregg Easterbrook, an environmental commentator who has a long record of opposing alarmism, put it this way: “All of the world’s major science academies have said they are convinced climate change is happen[ing] and that human action plays a role.”

The concentration in the atmosphere of carbon dioxide has increased markedly during the past 150 years, with fossil-fuel burning being one of the main contributors (deforestation is another). Estimates are that humans have been responsible for almost a 40 percent increase in atmospheric CO2 concentration during the past two centuries. It is indisputable that atmospheric carbon dioxide has increased; it makes complete sense that the effects of this increase would lead to a warming of Earth’s surface. The reason is that greenhouse gases enhance the natural greenhouse effect by absorbing outgoing infrared radiation and re-radiating some of it back toward the surface, thereby warming it. This is not a liberal invention; it’s physics.

Where there’s a good deal more uncertainty is in future climate projections and what needs to be done – which I’ll address in my next post.