A bleak report released by the UN's Intergovernmental Panel on Climate Change, the fifth in a series, makes several things clear: climate change is well underway, it will affect food supplies and global stability, and politicians — particularly American ones — should be embarrassed at their inaction.

To be fair, that last point is subtextual, an unavoidable conclusion from the worst parts of the report. It's hard not to see that the ongoing failure to address climate change is the biggest geopolitical misstep of the last 80 years.

What the report says: Climate change is happening.

The New York Times has the best summary: "ice caps are melting, sea ice in the Arctic is collapsing, water supplies are coming under stress, heat waves and heavy rains are intensifying, coral reefs are dying, and fish and many other creatures are migrating toward the poles or in some cases going extinct." Also: crop growing patterns are being disrupted, threatening the food supply. Oceans are rising, thanks to warmer sea temperatures and melting ice. They're growing more acidic thanks to increased CO2 absorption, which can dissolve the shells of shellfish. The thawing permafrost means more organic material is decomposing, releasing even more carbon dioxide into the atmosphere.

By the end of the century, 75 percent of food-growing regions are expected to see reduced crop yields — under a conservative warming scenario.

These things are happening currently, on "all continents and across the oceans."

The effects of climate change — the climate change that is already happening — will be disproportionately felt by the global poor, as the Guardian reports. In part, that's because the areas that will experience severe weather and flooding are disproportionately poor (see the Times' series on Bangladesh). The poor are already more likely to be on the tipping point already, meaning that displacement due to flooding or a lack of readily available food. And they are more likely to live in politically unstable areas. "[C]hanges to the Earth's climate are fuelling violent conflicts," the Guardian writes. "The UN for the first time in this report has designated climate change a threat to human security." See: Darfur.

There are multiple paths forward.

"Large magnitudes of warming increase the likelihood of severe, pervasive, and challenging impacts," the report states. But those large magnitudes of warming are avoidable. For the most tangible effect of climate change, increased warming, the report maps two warming scenarios.

The map at left shows how temperatures will increase by the end of the century under the low-end "representative concentration pathway" — that is, an estimate of the amount of carbon dioxide in the atmosphere at that point that is the lowest the ICPP considered. At right, the high-end estimate. The axis at left shows CO2 concentrations; right now, the level is hovering around 400.

What do those two paths look like in practice? The graph at right, from Wikipedia summarizes the IPCC analysis. The low-end estimate assumes that carbon dioxide levels in the atmosphere peak near 450 parts-per-million and decline beginning at the middle of the century. The high-end estimate assumes that the concentration of emissions continues unabated.

Climate change is happening. The question is how bad we allow it to become.

Political inaction is embarrassing.

As ThinkProgress' Joe Romm points out, the IPCC assessment offers a conservative estimate of what to expect, only assuming that warming will hit 7 to 9 degrees Fahrenheit over time. That will help make the case to world leaders that the report is not some bizarre, extreme environmentalist rant; rather, it's a thoughtful, sober assessment of what's to come and what can be avoided.

Perhaps the most powerful chart in the report (the lengthy introduction of which is here) is the one below. It shows three eras: climate change in the present, the era of change to which our current emissions have committed us (lasting the next 25 years or so), and the era that we can still affect.

The question is whether or not we will sidestep that third stage, with its multiple adverse effects. So far, there's been little indication that we will. The political challenge of climate change is not that world leaders don't accept that it's happening, it's that there's little impetus to actually act on it.

Leaders in countries that will be disproportionately and immediately affected by climate change are largely not the countries responsible for the greenhouse gas emissions that have caused the problem, as The Wire noted last week. Countries that are the heaviest polluters — the United States, China, and the European Union, in particular — see little short-term economic benefit in curbing that pollution.

The difference between the global reaction to the hole in ozone layer, which went from an observed problem in 1985 to an international solution in 1987, was threefold. First, the producers of CFCs, the chemical responsible for the hole, didn't have enormous economic clout. Second, they could quickly switch CFCs with substitutes. And, third, the national economy didn't rely on CFCs to operate.

The number one creator of greenhouse gas pollution in developed countries is burning coal for the production of electricity. As more countries enter the global middle class, that problem continues to grow, with 1,200 new coal plants planned in 59 countries over the short-term. That's a path that will lead to the worst-case CO2 projections, not the best.

The United States could have taken an active role in curbing coal-burning and greenhouse gas emissions. It could have exercised its international economic power to encourage a global transition away from burning coal. It could have quickly set new emissions standards for coal-burning power plants — something mandated by the Supreme Court under George W. Bush and still not finalized for existing power plants even today. But the American democratic process failed on this. "Politically speaking, it’s always easier to shell out money for a disaster that has already happened, with clearly identifiable victims," The New Yorker's James Surowiecki wrote after Sandy, "than to invest money in protecting against something that may or may not happen in the future." So no action has been taken.

The IPCC report is its fifth. The first came out in 1990; a second, reflecting updated science, in 1995. Another in 2001, another in 2007. The most recent iteration is simply the most bleak, the most updated predictions of how bad it will get. Will this be the one that prompts the United States Congress to move to action, to change our country's policies to try and stay on that lower emissions path while using our international power to change the behavior of other countries? No. It probably will not. It’s always easier to shell out money for a disaster that has already happened.