Two dozen scientists, authors, and world and national figures answered two questions: What is your greatest worry about climate change? What gives you hope? Here are some of their answers, condensed for space.

JANE LUBCHENCO, Former administrator, National Oceanic and Atmospheric Administration

I worry about oceans becoming more corrosive, decimating both fisheries and coral reefs. Oceans have already become 30 percent more acidic since the beginning of the Industrial Revolution; if business-as-usual carbon emissions continue, oceans are likely to be 150 percent more acidic by the end of this century. Yikes!

I take heart in knowing that social change can happen very rapidly once a tipping point is reached, that young people are bringing new passion and creativity to the issue, and that climate change is being seen increasingly as the moral issue it is.

TENZIN GYATSO, 14th Dalai Lama of Tibet

The worst possible aspect of climate change is that it will be irreversible and irrevocable. Therefore, there is the urgency to do whatever we can to protect the environment while we can.

When I was young, even I did not really think about the environment, nor did I hear much about it from others. Today, more and more people are trying to take action. We are beginning to look at this planet as our only home, and I am hopeful that this will lead to the generation of a genuine sense of universal responsibility. We can do this.

MICHAEL R. BLOOMBERG, Former mayor of New York City and special United Nations envoy for cities and climate change

Something like 90 percent of the world’s cities are on coasts, and in most places, the most vulnerable people in those cities will feel the worst impacts. We have a responsibility to do something about that. We can’t afford to sit back, cross our fingers and hope for the best.

A tremendous amount of progress is being made by cities all around the world. Cities account for some 70 percent of the emissions that cause climate change, so together they can make a big difference. In New York City back in 2007, we set a goal of reducing our carbon footprint 30 percent by 2030, and we got to a 19 percent reduction in just six years.

Mayors have powers they can use to address climate change immediately. They have control over many of the things that create emissions — like transportation and buildings — and they can invest in infrastructure. They’re not interested in turning the issue into a big political fight. They’re the ones most directly responsible for people’s safety and welfare — and they recognize the dangers of inaction.

JEFFREY SACHS, Director, Earth Institute of Columbia University

The oil industry has lobbied Washington to a state of paralysis, and as is so often true, greed is at the root of the crisis, with the politicians getting in line to feed at the oil trough. The climate deniers are not the real problem. Their transparent propaganda and misdirections are laughable; their scientific ignorance is impossible to miss. The real problem is the cowardice and greed of those who absolutely know better, both in government and industry.

We are living in an age of technological breakthroughs that could transform the world economy to a low-carbon energy system by midcentury. Solar, wind, geothermal, carbon sequestration, safe nuclear energy, and energy efficiency are all part of the mix. The oil industry should cooperate, rather than faking it or dodging it as until now.

BARBARA KINGSOLVER, Novelist; author of the memoir “Animal, Vegetable, Miracle”

My fear: Catastrophic extinction. We don’t get to make natural laws. Natural law made us, and it ultimately will unmake us. What makes me very sad is that we’re going to take so many species down with us.

My hope: We in the United States finally seem to be coming to the table after decades of either denial or argument. It seems as if denial as a political strategy has run its course and that we are stepping up to our responsibilities. I hope that’s true.

ALAN I. LESHNER, Chief executive, American Association for the Advancement of Science

Ideology and intuition sometimes appear to be trumping science. So people deny the evidence even as it increases. I fear that the pace at which the public understands that the climate is changing, and puts pressure on the political system, will be too slow.

We are seeing that communicating scientific knowledge has had an effect, and that makes me happy! The deniers have less and less credibility as the public understands the scientific consensus more and more.

NEIL DeGRASSE TYSON, Director, Hayden Planetarium

I find that to worry about things is to invest emotional energy in ways that do not lead to change. Always better to do something about a problem than to worry about it.

What I expect will happen in the coming decades is that beachfront real estate, some of the most expensive in the housing marketplace, will become overrun by storm surges with enough frequency that it will force the wealthiest class (who might have previously been in denial of the phenomenon) to recognize the problem and take action, actions they can take since they are typically captains of industry and are in power and in control.

JERRY BROWN, Governor of California

A huge challenge of climate change lies in the fact that for its solution, countries all over the world must collaborate in ways that are entirely unprecedented.

Each nation-state has to be fully engaged and take decisive steps outside the conventional economic comfort zone. And that requires more statesmanship that is currently in evidence in any of those countries. The mythology of the market and economistic view of life has to be transcended so people understand that a decent and sustainable quality of life requires a very different philosophy than the one that governs contemporary societies.

Here in California, we’re leading the nation in the economic recovery and the creation of jobs, and we are pioneering climate change strategies across a broad front. We have a robust cap-and-trade system. We have a goal of one-third renewable energy in the electricity sector; we’re already at 22 percent. We have the strictest building standards in the world. We have a goal of over a million electric vehicles; we’ve got our first 100,000! We have a certain momentum in California. There are other states where this is also true.

JAMES E. HANSEN, Climate scientist, emeritus director at NASA Goddard Institute for Space Studies

The reason it’s a really dangerous situation is that the climate system does not respond quickly to the forces we apply to it. That means that we have not witnessed the impact of the gases we’ve already added to the atmosphere. We’re waiting for the public to see enough to demand effective government response.

The public doesn’t see that much yet, but there’s more in the pipeline. We are pumping energy into the ocean at a rapid rate; that energy is accumulating, and its biggest impact is going to be on ice shelves. The sea level will go up many meters. That means all coastal cities will be doomed if we stay on fossil fuel business as usual.

The upside is that the only policy that will work is making the price of fossil fuels match their cost. We have an organization determined to focus on exactly that issue: the Citizens Climate Lobby. It’s growing rapidly. Things are changing. But not fast enough.

MARIO J. MOLINA, Co-recipient of the 1995 Nobel Prize for his research on the chemistry of the ozone layer

What worries me most is the irrationality of certain interest groups preventing society from addressing the problem. Republicans in Congress are preventing action on an efficient solution such as a carbon tax.

There is a solution at hand. It doesn’t cost as much as the deniers claim. The Montreal Protocol [on ozone depletion] showed that you could solve such global problems. It would have been much more expensive not to solve it.

ELIZABETH KOLBERT, Author of “The Sixth Extinction: An Unnatural History”

For the last 10,000 years or so, the climate’s been relatively stable. But if you start imagining a world with a constantly changing climate, one where, say, rainfall patterns shift dramatically every few decades, then you begin to realize how dependent we all are on that stability. And the world we’re creating is that constantly changing one.

So I worry about just about everything, starting with the basics. There are 7.2 billion people on the planet right now, and we all need to eat.

Hopefulness or a lack of it is really not the issue here. We’ve already caused a lot of damage; there’s a lot of warming that’s in effect baked into the system. We’re capable of causing a great deal more damage, and we’re also capable of limiting that damage. That’s the choice at this point, and we need to face up to that.

J. MARSHALL SHEPHERD, Former president, American Meteorological Society

It bothers me that people think there’s a big debate in science when there isn’t. Being concerned about climate change is not some whim. When I go to the mall or to Walmart, people ask, “Do you really believe in climate change?” That’s like asking, “Do you believe in gravity?” I mean, the science is clear.

What gives me optimism is that many of the people who question the science are of an older generation. The kids get it. When I go to my children’s Scout meetings or when I talk to students on campus, they get beyond the misinformation and politics.

THE REV. MITCHELL C. HESCOX, President, Evangelical Environmental Network

Climate change is the greatest moral challenge of our time because it impacts every single soul in the world. In the conservative Christian world in the United States, we’ve gotten caught up in political partisanship. I’d like to see climate change as a Christian issue and not a partisan issue.

We are the stewards of God’s creation. We believe that the earth’s creation belongs to God and that we are charged to care for it.

When we started this [network] five years ago, we had 15,000 people we regularly communicated with on this issue. Today it approaches 400,000. It means that we’re starting to overcome the partisan divide and the tide is slowly turning.

DIANA H. WALL, Director, School of Global Environmental Sustainability, Colorado State University

What keeps me up, the thing that really drives me nuts, is that the rate of change is so fast. I work in one of the most extreme ecosystems on Earth: the Antarctic Dry Valleys. It is the coldest, windiest, driest place on Earth. We’re seeing warming events and very sunny events there, and this is causing a change in the organisms I study. These species have adapted to the conditions there. We don’t know what the impact will be on them or us.

My students give me hope.

MARTIN REES, Astrophysicist, University of Cambridge

I have a lot more fears than hopes. One aspect that particularly troubles me is that economists tend to underprioritize efforts at mitigation of atmospheric carbon, because the really serious downside of inaction won’t be experienced until the 22nd century and beyond. If action is delayed, it may then be too late to avoid irreversible runaway changes.

We shouldn’t discriminate against our fellow humans on grounds of date of birth. The lifetime welfare of the newborn should rate as highly as that of the already middle-aged. Indeed, many philosophers would assign equal value to the rights of those not yet born.

For them, foreclosing the potentialities of all future generations would be so catastrophic that we should strive to reduce even the tiniest probability that this could happen.

CHRISTINE TODD WHITMAN, Former governor of New Jersey and former administrator, Environmental Protection Agency

What keeps me up at night are people who talk in absolutes. It’s the people who say “humans cause it” or “people have no role in it,” full stop. Science is not exact and the truth is in between. By taking the extreme position, they give an opening to the other side, and then people stop listening.

What gives me hope is that there are signs that the American people are beginning to relate some of the frequent weather extremes to climate.

Since 1980 our economy has grown, our population has grown and our energy use has grown, and yet our overall pollution has gone down. We are perfectly capable of implementing environmental regulation without stopping economic growth.

KATHARINE HAYHOE, Director, Climate Science Center, Texas Tech University

What troubles me as a scientist is the potential for vicious feedbacks within the climate system. The warming that we cause through all the carbon we produce could cause a series of cascading impacts that could lead to a much greater warming. The more carbon we produce, the higher the likelihood of these unpredictable risks.

What makes me hopeful are people. I’ve been working with cities, states and regional transportation councils, and none of them have to be convinced of the reality of this problem. I was sitting next to an assistant city manager for Dallas, a town not known for being green, and she blew me away with her list of amazing things Dallas has done to save energy. People are preparing for change.

MARGARET ATWOOD, Poet and novelist, author of “The Year of the Flood”

The most worrisome thing is the potential death of the ocean. If it dies, we die.

What gives me hope is that more and more people are aware of the dangers we face, and many smart people are at work on solutions. Our smart brains got us into this. Let’s see if they can get us out.

FREEMAN J. DYSON, Theoretical physicist, Institute for Advanced Study

What worries me is that many people, including scientists and politicians, believe a whole lot of dogmatic nonsense about climate change. The nonsense says that climate change is a terrible danger and that it is something we could do something about if we wanted to. The whole point is to scare people, and this has been done very successfully.

Climate has always been changing, and climate has always been lousy. It has always been a background to existence that on the whole we’ve learned to cope with pretty well. What I feel happy about is that there are a lot of ordinary people with common sense who don’t believe the nonsense.

MARLENE MOSES, Nauru’s ambassador to the United Nations, chairwoman of the Alliance of Small Island States

When I go home and look at the deteriorating situation there — increased droughts, the ocean washing away the coast — I can’t help but be fearful for what the future may hold for Nauru’s children and grandchildren. How will they adapt? Will the international community be there for them? These are most distressing questions to which I don’t yet have answers.

GLORIA STEINEM, Co-founder and former editor, Ms. Magazine

Thinking about climate change used to give me images of the sun burning down and icebergs melting — horrific, but also impersonal and far away. Now I have intimate fears of storms and floods that drive us off this island of Manhattan, and fires that send thousands fleeing — in other words, just an acceleration of what we’re already seeing.

Like millions of others in public opinion polls, I’m willing to lower my standard of living to help create a turning point. We’re waiting for a practical, coordinated, understandable set of instructions that counters the Kochs, the deniers, the profiteers. Meanwhile, we try to do whatever we can.

Somehow, I find comfort in the idea that the earth is a living organism with a will of its own. The global women’s movement gives me hope because women are trying to take control of their own bodies and reproduction, which is even more basic than production. Everything we know says that when women can decide whether and when to have children, growth slows down to a little over replacement level. And that would be the single biggest long-term relief for the environment.

MARY ROBINSON, Former president of Ireland, former United Nations High Commissioner for human rights

I’m a grandmother with five grandchildren. What will they say about what we did or didn’t do?