It's a huge day for climate policy. Last night, United States and China announced a joint plan to limit greenhouse gas emissions: the US will cut its emissions by over a quarter beneath 2005 levels by 2025, while China pledges to stop emissions growth by 2030. This is a big deal — it breaks the stalemate that had been holding up international climate negotiations.

This is significant for the effort against climate change and for US foreign policy, but there's another issue for which it matters a great deal: global poverty. This new agreement with China, together with Obama's plan announced in June to regulate emissions from existing power plants at home, might be Obama's greatest contribution to the fight against global poverty. Climate change is bad for everyone. But it's particularly bad for the world's poorest people.

Climate change hurts the global poor the most

Standard and Poor's, the credit rating company, published a report in May assessing the risk each country faces from climate change. You'll notice the more vulnerable (redder) countries cluster in Asia and Africa, while the better off (greener) countries are almost all in North America or Europe:

The cost to the developing world of unmitigated climate change will be staggering. Assume the globe warms by about four degrees celsius, which is easily possible absent major emissions reductions. Those notorious hippies at the World Bank suggest that this could lead to massive increases in disease, extreme storms, droughts, and flooding. The effects will be so massive, according to the bank's President Jim Kim, that they could roll back "decades of development gains and force tens of more millions of people to live in poverty."

This isn't theoretical: we're already seeing the harm climate change is doing to the global poor. The World Health Organization estimates that 150,000 people are killed by climate change's effects annually — a number that's obviously going to rise significantly as the earth heats up. That's about 3/4 the total death toll of the Syrian civil war — every year. And once again, poor countries are hit by far the hardest:

See those dark red splotches in south-central Africa? That's climate change's toll on some of the world's least advantaged countries, right now.

The big causes of climate inequality: geography and poverty

There are basically two reasons why poor countries have it so rough: geography and poverty. Most of the red countries on the S&P map lie near the equator, where climate change-caused storms, flooding, and droughts will be more intense.

Poverty makes these effects much worse. You'll notice, for instance, that Indonesia is red on the S&P map while Australia, its much wealthier neighbor, is green. Poor countries tend to have more poorly constructed homes, fewer resources for emergency response and relocation after sea level rise, and weaker health care systems ill-equipped to deal with increased rates of tropical disease. Imagine Hurricane Katrina levels of flooding in a city like Dhaka, Bangladesh — home to 15 million people.

Actually, we don't need to imagine: we know from "normal" extreme-weather events that poor countries have it the roughest. From 1980-2007, only 15 percent of hurricanes, typhoons, and the like happened in low-income countries, but 68 percent of people killed by these storms died in poorer nations.

For these reasons, a group of the world's 49 poorest countries are already lobbying wealthier nations — which are responsible for a disproportionate share of carbon emissions — to commit to tighter emissions standards at a 2015 summit in Paris.

Why American and Chinese action is such a big deal for international poverty

Obama's June regulations on power plants were America's unilateral contribution to that effort. 17 percent of the world's emissions come from the United States. The regulations are designed to cut that number significantly, ideally reducing the United States to 17 percent below its 2005 target.

Wealthy countries have done a pretty poor job addressing climate change as an inequality issue

This will cost the United States some money, though it's not clear how much. One way to think about this is a transfer payment: America, a rich country, is spending to try to save the world's poorest from the worst effects of climate change.

Again, warming is pretty bad for everyone, so the regulations aren't totally altruistic. But the world's poorest will be hurt the most, and they're the people who can least afford to be hurt. The global gap in wealth is unimaginably large. To put it in some perspective: the poorest five percent of Americans have higher per-capita incomes than 68 percent of the world.

So far, wealthy countries have done a pretty poor job addressing climate change as an inequality issue. Rich countries promised $100 billion per year to help poor countries deal with the consequences of global warming, but most of that money hasn't materialized. This is doubly infuriating for leaders of poor countries, as the global rich got that way by using technologies that created the climate crisis in the first place. The same carbon-powered economic growth fuelled Western colonialism.

That's why the real goal of any climate regulation is to effect global change — and why the new China announcement is such a big deal. China and the United States are the world's two largest emitters: it would be impossible to get a handle on climate change without serious action on their parts.

The current deal isn't enough to avert catastrophic change on its own. But it does break an impasse between the US and China, where each major power was a bit skeptical of any international deal on climate change unless the other committed first. The bilateral agreement not only shows that America and China can cooperate on climate change, but also lays the groundwork for future, more far-reaching negotiations.

That includes a full international deal. Throughout 2015, the international community is conducting talks to create a new global plan to reduce emissions and tackle climate change. Lack of agreement between the US and China could have hamstrung the talks — and it still could. But the new agreement shows real commitment by the leading emitters to tackle climate change, indicating that US-China competition may not be as much of a barrier as previously thought. It also signals to the rest of the international community that negotiations are worth taking seriously.

So the US-China agreement will build global support for a campaign against one of the most important causes of global poverty and inequality. That's the big-picture story behind yesterday's move. The biggest question going forward is whether it'll work.