Last week at the North American Leaders’ Summit featuring President Obama, Canadian Prime Minister Justin Trudeau and Mexican President Enrique Pena Nieto, the three nations announced a goal of generating 50 percent of North America’s electricity from “clean” sources by 2025. It’s a laudable goal, but it naturally raises a question — how exactly, in the United States, do we get there?

A closer look at what the White House and its counterparts actually mean by this proposal shows that for the United States the goal relies on far more than an ongoing boom in wind and solar. It rests substantially on hydropower and energy efficiency gains and also includes under the definition of “clean energy” two technologies that are less than popular in the environmental movement — nuclear energy and carbon capture and storage. Yet it is hard to say that nuclear and CCS are booming in this country; it would be more accurate to say that both are struggling at the moment.

Thus, the new goal raises a serious question of precisely how we are transitioning toward a future in which far more (and, eventually, all) of our electricity is generated without emissions of carbon dioxide to the atmosphere — and how different types of power generation will slice up this new pie.

Let’s start in the easy place. Wind and solar are growing in the United States (and elsewhere), and a doubling of their U.S. generating capacity (or even greater growth than that) by 2025 isn’t hard to imagine. We know we are going to be getting a lot more of our future electricity from wind and solar than we do now.

That’s a very good thing — but even a doubling of wind and solar likely wouldn’t be enough to get the United States to 50 percent clean electricity by 2025. The trouble is that in 2015, these sources contributed a little more than 5 percent of all U.S. electricity. They are starting from a relatively low level of penetration, albeit with high growth rates.

For just this reason, nuclear’s inclusion in the North American plan is a mathematical necessity — it provides about 20 percent of the United States’ overall electricity and a far larger percentage of its carbon-free power. The centrality of nuclear arises in part because unlike wind and solar deployments, nuclear plants generate electricity almost continuously, often generating above 90 percent of their maximum capacity in a year. Solar, in contrast, is much more intermittent. So is wind.

However, unlike wind and solar, the nuclear industry is not in great shape right now in the United States and hardly looks poised for much growth out to 2025.

In June the large California utility Pacific Gas and Electric announced plans — in the form of an agreement with labor and environmental groups — to close the Diablo Canyon nuclear plant, whose two reactors provide a stunning 9 percent of the entire state’s energy, by the year 2025. The vast amount of electricity currently generated by Diablo Canyon would be replaced, the company said, with wind, solar, batteries and more energy efficiency.

It’s just the latest indicator that the future is cloudy for nuclear. Five U.S. reactors closed in 2013 and 2014, and Diablo Canyon joins the list of numerous other planned closures in coming years.

The United States is also expected to add five new nuclear reactors in coming years — one, Watts Bar 2 in Tennessee, is already generating electricity and has sent some to the grid — but it seems poised to end up with roughly the same or perhaps less nuclear generation overall in 2025. Whether the situation worsens even further for nuclear will depend on economic factors, like the price of natural gas, that will be hard to forecast.

And then there’s carbon capture and storage — which basically refers to techniques to keep the carbon emissions from coal- or gas-fired electricity generation, or other industrial processes, from reaching the atmosphere and instead channel them into reservoirs in the ground. Sometimes, that also involves using the carbon dioxide to make a little extra income along the way through enhanced oil recovery.

CCS is still a fledgling and quite variable technology, with only a small number of projects around the world operating in the electricity generation sector. Yet it plays a key role in many scenarios that scientists and analysts consult to study how the world can solve its carbon problem while still providing electricity to a growing global population.

“The [Intergovernmental Panel on Climate Change] models show that CCS is critical to keeping our global temperature increases within 2 degrees Celsius of pre-industrial levels,” notes Fatima Ahmad, a fellow at the Center for Climate and Energy Solutions (C2ES) who has focused on the CCS space.

Indeed, agreeing with this perspective, the International Energy Agency recently found that “CCS needs to increase by an order of magnitude in the next decade” if we are to keep global warming below 2 degrees C.

What’s more, CCS is one key element in a technological combination dubbed “bioenergy combined with carbon capture and storage,” or BECCS, that is the leading contender right now for how the world will someday achieve “negative” carbon emissions — or, in other words, pull carbon dioxide back out of the air. We’re already so far gone in the climate change arena that many realists believe a technology like this will be a future necessity.

And yet for CCS, if anything, the story is even more challenging than it is for nuclear.

Last year, the Obama administration cut funding for the FutureGen 2.0 project in Illinois, which was supposed to be a key demonstration of the technology. This year, meanwhile, the Energy Department reportedly decided to suspend funding for another CCS project in Texas.

And now, the New York Times is out with a highly critical report about the Kemper Plant in Mississippi, which the paper says has suffered from major cost overruns, though it is still expected to begin operating this year. (Southern Co. has disputed the Times story).

All of which is just part of a broader picture of a CCS industry that could play a key role in fighting climate change — but has a long way to go before that can happen.

The challenges are numerous, explains Jeff Erikson, general manager for the Americas region at the Global CCS Institute. “The economics are challenging, on a project basis, unless there is a regulatory requirement or an income stream, or significant government support to make the economics work,” Erikson said. “But right now, why put CCS on a power plant when there’s no mandate to do so?”

But Erikson thinks there will still be growth of CCS in the United States and globally and that it will in the future be applied not only to coal-burning power plants but also to natural gas plants and many industrial applications such as steel and cement plants, which also produce carbon dioxide emissions. (The Global CCS Institute currently lists 15 “large scale” CCS projects that are underway around the globe.)

CCS and nuclear have much in common — and not just their sometimes contradictory relationship with the environmental movement. They also differ from wind and solar in that plants tend to be gigantic, billion-dollar projects, making them much more difficult to finance than more flexible and often smaller-scale renewables.

Moreover, both CCS and nuclear would benefit if we put a price on carbon, making its emission to the atmosphere more expensive whether the source was cars or electricity generating plants.

But that hasn’t happened yet, and the politics of it remain exceedingly difficult. It is notable that, climate change champion though she is, Hillary Clinton has not embraced a tax on carbon among her suite of proposed climate policies, despite its widespread support from economists.

In the end, the issue is this: Without a carbon tax or cap and trade system — and with the legally troubled Clean Power Plan not yet operative — what we are seeing is that wind and solar have managed to boom anyway, and started a period of rapid growth. This has been thanks in part to state-level renewables policies in the United States and government tax incentives, but also to some smart business innovators and some advantages inherent in the technologies. But other technologies that many analysts believe will be key to a less carbon-intense future are not faring as well right now.

Should that worry us? Some researchers, like Mark Jacobson of Stanford and his colleagues, argue that the United States can be powered entirely with wind, solar and water batteries by 2050 (assuming we enact the right policies to get there, that is). Obama’s economic advisers, meanwhile, recently published a study suggesting that despite their intermittent nature, it will be possible in the future to integrate more and more wind and solar onto the grid thanks to advances in batteries and other technologies.

Still, there is hardly a consensus about the right way to reach low-carbon energy goals at the moment, and there are also many who argue that nuclear and CCS will also be essential, for many of the reasons outlined above. It’s just that their future seems murky, at best.