Los Angeles is not one of those gracious cities that are kind to visitors. Its urban surprises, rather than expanding one’s horizons, often involve unpleasantness. Last summer I had a visitor from New York. A few days into his stay the mercury hit 95 degrees, and he had the understandable impulse to visit the Santa Monica beach. We fought heavy traffic all the way there. What should have taken 15 minutes took more than an hour. We tried to entertain each other with conversation. He’s a committed environmentalist, and the traffic was a perfect excuse to talk about L.A.’s famous orientation toward the automobile. Surveying the 10-lane freeway, he said, “Seeing L.A. reminds me how difficult it is to stop burning fossil fuels. How do you change habits and even begin to address climate change when so many lives have been built around the car?” It was a tough question, and as we sat silently contemplating it, I knew his opinion of the city was sinking. He gazed at the idling cars emitting tons of heat-trapping carbon dioxide. I made a lame attempt to emphasize the positive. “Did you know that in 2008, L.A. County voters passed a ballot measure raising the sales tax to pay for public transit projects? Right now they’re using the funds to build a light-rail line from downtown to Santa Monica.” “Really?” he replied. “When’s it going to be done?” There was a moment of silence. “I think 2016,” was my feeble answer. I knew it was a setup for his snarky response: “Really? How many years before you even have the option of a low-carbon trip to the beach? At that rate, we might as well just build seawalls and forget about climate change.” I laughed politely.

My houseguest had a point. Even if the entire world were to imitate California and Europe, it would still require a long time to slow increases in greenhouse-gas emissions, let alone reverse them. Renewable energy could become cheap enough to be competitive with fossil fuels without market intervention. But even if there were breakthroughs tomorrow, they would still take years to evolve from concept to marketable products. The most rapid shift possible from fossil fuels to renewables would probably take at least a decade or two. So we’ll be emitting greenhouse gases for some time no matter what choices we make now about energy. Couple that with the fact we’re already committed to some change from our previous greenhouse-gas increases, and it seems clear: Significant climate change is impossible to avoid.

When we finally arrived at the beach, my friend tore off his shoes and raced to the water. He got out far enough to submerge his calves before running back, yelling, “That water is freezing. It’s so hot out. Why is the water so cold?” In front of most people, those words would have been a rhetorical shake of a fist at the sky. But as he realized he had uttered them in front of a climate scientist, I could see the weary amusement sweep across his face. “Oh no, you’re actually going to answer that question.” “Of course,” I said, smiling. “When the sunshine hitting the northern hemisphere undergoes its annual increase from winter to summer, the temperature of the ocean increases a lot more slowly than the land. This is because so much more heat is needed to raise the ocean temperature one degree. So the temperature of the ocean ends up peaking at a lower value than the land. That’s the most important reason you’re so shocked about the water temperature. But ocean currents also make this patch of the Pacific especially cold.” He tilted his head down and looked up at me. “Okay, well now we know why this beach isn’t as fun as it could be.” This time I laughed for real.

The relatively slow ocean warming in response to increasing sunshine does more than disappoint beachgoers. It has an exact parallel in climate change, creating a lag in warming. In computer-based simulations of the climate system’s response to increasing greenhouse-gas concentrations, the ocean receives the same amount of extra heat from the greenhouse-gas buildup as the land does. But it registers a much slower warming. Likewise, as greenhouse-gas concentrations have increased over the past century, the accompanying temperature increase has generally been smaller over oceans than over continents. This effect works to our advantage because it gives us more time to adapt to a changing climate—especially in coastal areas, where climate is influenced by the ocean. But it’s also symptomatic of the irreversibility of climate change. The ocean eventually has to adjust fully to enhanced heat input. So the global warming we see at any given time doesn’t fully reflect what will ultimately materialize as a result of the previous greenhouse-gas buildup. This leads to a disconnect between the current state of the climate—which probably has the greatest influence on our perceptions of climate change—and the eventual climate change we are already committed to, thanks to our past burning of fossil fuels.

There’s momentum behind climate change for another reason, but this one is related to the inertia of human habits rather than to physics. It’s impractical for us to stop burning fossil fuels right away, even if we collectively decided to. California, much of Europe and other localities are creating incentives to transition away from fossil fuels. In these jurisdictions, efforts are under way to lower carbon footprints. The light-rail line to Santa Monica is an example. But as I noted to my skeptical houseguest, even though Angelenos have largely embraced new rail transportation projects, this line won’t be finished until 2016. And it’s one of the first elements of a regional rail network that will take decades to complete.

Most people seem to grasp the inevitability of climate change. Ask a climate scientist what question he or she usually gets at cocktail parties, and it’s something along the lines of “Based on your research, are we totally screwed?” It may also be the underlying reason why some people deny climate change: The problem may seem too insurmountable to grapple with. For sanity’s sake, we might as well change the subject to something we can grapple with.

The inevitability of climate change was underscored for me when my UCLA research team and I completed a study on what climate change will mean for Los Angeles in the middle of this century. Our research shows that if greenhouse-gas emissions are curtailed over the coming decades, the most likely warming in the region will still be 70 percent of what would occur if greenhouse-gas emissions continue to increase. Climate scientists work with standardized scenarios of greenhouse-gas emissions to predict future climates. We worked with a scenario of reduced emissions we nicknamed “mitigation” and a scenario of rising emissions we called “business as usual.” Both scenarios result in significant regional warming: about 2.9 degrees Fahrenheit under the mitigation scenario and 4.2 degrees under business as usual.

Put differently, immediate and far-reaching global measures to reduce our carbon footprint over the coming decades would reduce midcentury warming in Los Angeles by only 30 percent. When we examined other aspects of climate change in the region—such as changes in snow, water resources, winds and wildfire—we reached the same conclusion: Reducing greenhouse-gas emissions attenuates climate change somewhat, but it doesn’t lead to dramatically different outcomes. A similar story can be told in other regions or for the entire planet. According to a recent report of the Intergovernmental Panel on Climate Change Fifth Assessment, the most likely midcentury rise in global sea level will be nine inches under the mitigation scenario and 10 inches under business as usual.

We have no choice but to think seriously about how to adapt to changing climatic conditions, no matter what we choose to do about emissions.

Given this conclusion, it may seem wise to simply adapt to climate change and be done with the whole issue, as my houseguest sarcastically suggested. We’re also not good at the long-term planning required to reduce greenhouse-gas emissions. This deficiency is especially apparent in the way we make economic decisions: Most people don’t plan adequately for retirement. A perennial complaint about corporations is their tendency to focus on quarterly earnings instead of long-term profitability. Governments borrow money to enact popular tax breaks or to spend, without giving thought to affordability in years to come.

When it comes to reducing greenhouse-gas emissions, the inability of governments to make long-term plans is perhaps the most crucial obstacle. Elections come every few years in Western democracies, where much of humanity’s emissions occur. Any politician who implements long-term reform will pay a price at the ballot box as soon as the reform begins to cause economic pain. And that politician won’t be around to see any political benefit when the reform bears fruit. Meanwhile, the state-capitalist regimes of East Asia, the planet’s other big carbon emitters, rely on consistent economic growth to perpetuate their authority. They don’t make structural economic changes easily either. With short-term thinking dominating politics in countries with the highest emissions, meaningful regulatory efforts to reduce greenhouse-gas emissions—such as a cap-and-trade system or a carbon tax—are rare.

There are few pragmatists when it comes to climate change, but if they were numerous enough to make their voices heard, their argument against action to reduce emissions might go like this: Humans are already on course to change the climate significantly, and our species is terrible at the long-term planning necessary to slow the increase in emissions. The best we can hope for is a robust adaptation plan, perhaps implemented locally and regionally, where natural-resource management is most nimble.

But let us consider results from climate change projections a bit further into the future. If we examine projected outcomes at the end of the 21st century rather than midcentury, the differences between the business-as-usual and mitigation scenarios are dramatic. In our Los Angeles study, under business as usual the region would experience another large increment of warming over and above what already occurs by midcentury. It would be warmer by a total of eight degrees. Three months of the year would have days when the maximum temperature exceeded 95 degrees, which would mean we’d have a new season of extreme heat. Southern California currently has significant snow at high elevations in the winter, but by the end of the century this snow would nearly disappear. This scenario also sees dramatic changes in water resources and in areas burned by wildfire. The ecological consequences are difficult to quantify, but it doesn’t take much imagination to realize that what’s left of the region’s natural landscape would be transformed beyond recognition, hosting different plant and animal species. Meanwhile, in the mitigation scenario, end-of-century temperatures are similar to their midcentury values. The other climate impacts are also comparable to those projected for the earlier period.

On the global scale, sea level would rise something like 17 inches under mitigation and 29 inches under business as usual. Some summer Arctic sea ice is projected to survive at century’s end if greenhouse-gas emissions are curtailed. But if we don’t curtail emissions, the ice will probably disappear. So our emissions choices will profoundly affect the planet’s condition at the end of the century.

Our pragmatic friends who propose adaptation as the only response have to defend the idea that the effects of climate change on the midcentury world can be considered in the moral equation but the effects on our end-of-century world cannot. At some point in the distant future we stop caring about the well-being of our species and rely instead on intervening generations to do the planning for us. But does it make sense to discount the interests of those who will be living at the end of the century, some of whom are already alive?

It’s also worth examining the other assumption of the pragmatist’s argument, that we are inherently bad at long-term planning. Consider a long-term project that involved profound political and societal change: the struggle for racial equality in the United States. Since the country’s founding, those who promoted racial equality knew it would take an effort extending well beyond the lifetime of any single individual. Among the country’s founders, some, including John Adams, explicitly opposed slavery. They accepted the constitutional arrangements that permitted it because they calculated that the language of equality and individual rights created a contradiction with the institution of slavery. They knew it would eventually have to be confronted. Within a few decades, they were proved right, and the country fought a civil war over slavery. Abraham Lincoln also understood the abolitionist project as part of a long struggle rooted in the promises of the founding documents. In spite of his opposition to slavery, he advanced the abolitionist cause only incrementally. Once slavery was eliminated and the struggle for social equality took center stage, civil rights campaigners also understood that the struggle would take time. The 2008 election of the first U.S. president with African ancestry was understood by most Americans to be an epochal moment. President Barack Obama’s election was an achievement built on at least two centuries of political and social change.

Similarly, the human role in climate took a few centuries to develop. Its roots go back at least to the beginnings of the industrial revolution in the late 18th century. Carbon emissions probably began to have a detectable impact on climate sometime in the mid–20th century. It took generations for the human influence on climate to emerge, and it will probably require generations to slow down or reverse that process. We’ve become accustomed to confronting problems with short-term fixes, and we keep waiting for a silver bullet to make our greenhouse-gas emissions disappear. When it fails to materialize, we throw up our hands and declare the problem beyond addressing. Looking back at the history of race relations in America, it’s easy to imagine how intractable a problem racial inequality must have seemed. For decades, many declared the status quo to be the only practical option.

Yet others kept chipping away at the status quo, and even though racial inequality and discrimination persist, those efforts led to a changed society. According to Martin Luther King Jr., “Faith is taking the first step even when you don’t see the whole staircase.” King would have found my houseguest’s cynicism about the light-rail line to Santa Monica entirely misplaced. The year 2016 is no further in the future now than 1966 was in 1963. And when King led the 1963 March on Washington, he knew it would take longer than three years to achieve a color-blind society. Some may believe the light-rail line to Santa Monica is too small a step or too far in the future to make a difference. But that belief condemns us to put off other small steps that would make a difference when added together.

We have to acknowledge the irreversibility of climate change. Like my disappointed houseguest, we can’t fight the physics of slow ocean warming, and we can’t wave a wand to make greenhouse-gas emissions disappear. Every region needs a climate-change adaptation plan for the coming few decades. But beyond that point, further climatic changes—perhaps to a point where the planet’s state is unrecognizably different—are not inevitable. If we care about the condition of that world, the only way to avoid those changes is to do what people have always done when faced with an overwhelming project: Get started.