The EPA outlines our choices on global warming: moderate disaster or major disaster

A trailer truck drives through flooded Sunrise Highway at Route 111 following heavy rains and flash flooding August 13, 2014, in Islip, New York.Andrew Theodorakis/Getty Images

This week, the EPA released a major report that tried to tally up the specific benefits to the United States if the whole world took action on climate change. Fewer deaths from heat waves, billions in saved infrastructure costs, and so on.

So far, so good. But a closer look at the EPA's report also reveals two other nuanced points about climate that are getting lost in the media coverage. They're worth emphasizing, because they give a clearer sense of what we're actually dealing with and what our choices are:

1) No matter what the world does on emissions, some amount of global warming is inevitable in the decades ahead. That will lead to all sorts of disruptions and dislocations, and we really ought to start planning and adapting now.

2) If the world does cut emissions drastically, those climate impacts will be less costly, and the risk of catastrophe goes down. But we also wouldn't see any major difference for decades. That's because there's a lag between when we put CO2 in the atmosphere and when its various impacts are felt. So when we talk about reducing emissions, we're mainly talking about benefits for the United States in 2050 and beyond.

The biggest benefits of tackling climate change show up after 2050

This chart, from the EPA report, illustrates the point well. It shows how precipitation in the United States would likely evolve under two different climate scenarios:

(EPA CIRA report)

The left column shows what climate models project will happen if global greenhouse gas emissions keep rising, relentlessly, through the century. The right column shows what models forecast if the whole world starts curbing emissions sharply, starting in the next few years, and meets its goal of limiting global warming to below 2°C (3.6°F).

In both scenarios, the American West can expect to suffer a significant drop in rainfall and increased risk of drought through 2025. California will face some painful adjustments no matter what. The major benefits of cutting emissions don't become truly apparent until 2050 and later: California would still be grappling with increased water stress, but it will be far less severe than what would have happened if emissions kept rising.

You see this again and again throughout the report. The chart below shows how average summertime temperatures in a few states would evolve under both the "reference" scenario (i.e., emissions go up endlessly) versus the "mitigation" scenario (i.e., we take serious action):

(EPA CIRA report)

Climate models expect Illinois and Maryland to get significantly hotter under either scenario — an important point for policymakers in Chicago or Washington, DC, thinking about how to protect people from deadly heat waves. Even if the world takes sweeping climate action, summers in Maryland will become about as hot in 2100 as they are in North Carolina today. The difference is that Maryland would avoid becoming as balmy as north Florida.

We'll have to adapt to serious sea-level rise no matter what

You can tell a similar story about sea-level rise:

(EPA CIRA report)

We've already loaded enough CO2 into the atmosphere to destabilize the ice sheets in West Antarctica and keep pushing up sea levels for many decades (and probably centuries) to come.

So at this point, our choice is between cutting emissions sharply and grappling with a couple feet of sea-level rise, or letting emissions rise without end and contending with even more sea-level rise. And, as the chart above shows, we wouldn't notice a huge difference until midcentury or so.

This next chart from the report makes the point in a different way. It displays the projected costs of sea-level rise and storm surges to coastal properties in 17 cities around the United States:

(EPA CIRA report)

Cities such as New Orleans, Tampa, Miami, Charleston, and New York are expected to face billions of dollars in flood damage and infrastructure costs even with serious mitigation — about $790 billion in all. This is slightly less than the costs if we just let sea-levels rise endlessly, but it's still a serious cost no matter what.

(A caveat here: The report explains that in both scenarios, modelers assumed every city would take timely and rational adaptation measures in as smooth a fashion as possible. That's ... not how things are likely to go down. And if adaptation goes less smoothly, the cost of unchecked global warming projects to be much higher than the cost of limited global warming.)

We're going to need more scenarios...

There's a lot that's valuable about this EPA report. It shows, in abundant detail, all the ways climate change could impose economic costs on the United States — from wilting our crops to damaging our coral reefs to wreaking havoc on our highways — and explains how we'll need to adapt. And, notably, it tries to quantify the benefits that would occur if the world managed to curtail greenhouse gas emissions.

That's an angle that's often missing from climate discussions. If we do all this work to cut greenhouse gas emissions today, what's in it for us? As Paul Slovic, an expert on decision-making and risk, told Lisa Bennett: "We have to have some sense of efficacy to motivate us to make changes in our lifestyle that are beneficial to the planet." The EPA report offers a first stab at quantifying "efficacy."

If there's a major shortcoming here, it's that the EPA report spends a lot of time comparing and illustrating two somewhat unlikely scenarios. On the one hand, you have an ultra-pessimistic vision of a world where emissions keep rising endlessly and we get 5°C or more of global warming by 2100. Then, on the other, you have this ultra-optimistic scenario in which every country in the world takes drastic action and limits global warming to 2°C or less.

More plausibly, we'll end up somewhere in the middle. It's looking increasingly unlikelythat the world can summon the political will and technological savvy necessary to keep global warming below 2°C. Quite honestly, we've nearly run out of time. But on the flip side, various countries like the United States, China, India, and Europe are at least takingsome significant actions to reduce emissions, so it's unlikely that greenhouse gases will keep rising endlessly, either.

Odds are, then, we'll end up facing a muddled choice between, say, 2.8°C of warming versus 3.6°C of warming (or whatnot). In that case, it will be useful to know the costs we'll be facing no matter what — and what the benefits of nudging down global warming another 0.2°C or 0.5°C are.