When the Bulletin of the Atomic Scientists was founded, climate change science was in its infancy. There were no global climate models, no supercomputers, and no satellite remote-sensing data. Only a few visionaries understood that man-made increases in the amount of carbon dioxide (CO₂) might cause large global climate changes. The definitive summary of atmospheric science in the decade after World War II was the Compendium of Meteorology, a large multi-authored volume published in 1951 by the American Meteorological Society. Its article on climate change, written by the distinguished British climatologist C. E. P. Brooks, reflects the prevailing expert opinion of that time.

The article began with this statement:

“In the past hundred years the burning of coal has increased the amount of CO₂ by a measurable amount (from 0.028 to 0.030 per cent), and Callendar (1939) sees in this an explanation of the recent rise in world temperature. But during the past 7,000 years there have been greater fluctuations of temperature without the intervention of man, and there seems to be no reason to regard the recent rise as more than a coincidence. This theory is not considered further.”

It is important to distinguish between facts and opinions. “Everyone is entitled to his own opinion, but not to his own facts.” Daniel Patrick Moynihan, who said that, was a wise and accomplished American politician, sociologist, and diplomat. Like everybody, I know some facts, and I have some opinions. I will first summarize the facts that we have learned from the science of climate change. Then I will give some opinions about what people and governments should do.

I think any rational response to climate change involves first knowing what the facts and evidence are. That is the province of science. There are many indicators measured globally over many decades that show that the Earth’s climate is warming. All the indicators expected to increase in a warming world are increasing, and all those expected to decrease in a warming world are decreasing. It’s definitely warming. It’s not a hoax. We observe it and measure it. The atmosphere is warming. So is the ocean. Sea level is rising. Ice sheets and glaciers and snow cover are shrinking. The amount of water vapor in the atmosphere is increasing. Climate change is real and serious. It’s not a remote threat for the distant future. It’s here and now.

It’s us. We’ve done the detective work. Just as wildfire experts can say whether a fire was caused by lightning, or by a campfire accidentally left burning, or by arson, we can show what is now causing the world to warm. Yes, some past climate change was natural, like ice ages coming and going, but the warming we have observed in recent decades is clearly caused by human activities. The evidence for that is overwhelming.

We now know what paces the beginnings and ends of ice ages. It is slow changes in the Earth’s orbit around the sun, that alter how sunlight is distributed over the surface of the Earth. We understand these changes in the orbit, and they take thousands of years to have an effect. They cannot possibly produce the climate changes that we observe occurring in just a few decades. Similarly, we can rule out other natural processes, such as changes in the energy in sunlight. They are quantitatively too small. Human activities, such as burning coal and oil and natural gas, are the dominant cause of the rapid climate change we now observe.

It hasn’t stopped. The warming is still continuing. We have good estimates of the global average temperature of the Earth’s surface from 1880 until the present; 1880 is about the time when we first had enough good thermometers located in enough places around the Earth to enable us to calculate a meaningful global average. The modern data is the most accurate. During about the last 50 years, from the 1970s until now, we know there has been a warming of about 1 degree Celsius, or 1.8 degrees Fahrenheit. All the warmest years on record are recent years.

The heat is mainly in the sea. Over 90 percent of the heat added to the climate system is in the oceans. How do we measure the heat stored in the ocean? That’s a good question, and it has a fascinating answer.

We now measure this increase in ocean heat content from an array of about 4,000 autonomous floats deployed throughout the world ocean under an international program called Argo. They have no engines and no propellers, but they move with the ocean currents at a depth of 1,000 meters, which is about 10 football fields. That’s where they are usually parked, and they are programmed to periodically sink another 1,000 meters lower and then rise to the surface while measuring quantities such as water temperature and salinity. They rise and sink by changing their volume. This is accomplished by pumping fluid into or out of a bladder on the float.

The floats store the measurements, and then, when they are on the surface, they locate by GPS and transmit the stored data via satellites to scientists. The change in float locations between one transmission and the next provides information on the currents at the depth where the floats were parked. As their batteries fail, the floats end their useful lives and must be replaced by new floats. The Argo floats have revolutionized our ability to observe the oceans. Argo data are available to everyone for free in near real time. New floats, allowing sampling to much greater depths, are now being developed.

Sea level is rising globally. We measure it from altimeters on satellites. There has been a rise of about 100 millimeters, which is about 4 inches, over the last 30 years or so. The rate of sea level rise is increasing too. Future sea level rise will be much greater than past sea level rise. The sea level rise is different at different locations on the Earth. Local sea level is affected by whether the land at that location is rising or sinking, and also by ocean currents, tides and other factors.

Ice is shrinking. Ice sheets on Greenland and Antarctica and almost every glacier worldwide are all shrinking. We know this from satellite missions called GRACE (the Gravity Recovery and Climate Experiment). These satellites determine the mass of the ice accurately by measuring the effect of the ice sheet on the Earth’s gravity. The technology of the GRACE missions involves two satellites in the same orbit which have a means of measuring the distance between the two satellites extremely accurately. This distance changes when gravity varies, which occurs when passing over ice sheets, and measuring the tiny change in the inter-satellite distance allows scientists to determine the mass of the ice.

Carbon dioxide absorbed by the ocean makes it more nearly acidic. That can affect the marine ecosystem and the food chain. The ocean absorbs some of the CO₂ that we emit into the atmosphere. Measurements show that the acidity parameter called pH is decreasing. Seawater is slightly basic (its pH is greater than 7), and we observe a shift towards neutral conditions (pH = 7), rather than to truly acidic conditions (pH less than 7).

Carbon dioxide amounts in the atmosphere, because of human activities, are now about 45 percent higher than they were in the early 1800s. We have good measurements of atmospheric CO₂ amounts over the last 800,000 years. These data come from analyzing fossil air trapped in ice in Greenland and Antarctica. They reveal large variations in CO₂ amounts, associated with ice ages starting and ending. Orbital variations pace the ice ages, causing the CO₂ amounts to change, and initiating a feedback that increases the magnitude of the temperature change.

The amount of carbon dioxide in the atmosphere now is the highest it has been in millions of years. The atmospheric CO₂ amounts in the even more distant geological past, many million years ago, sometimes have been even higher than at present, but the world was a very different place then, which was long before any human beings existed.

Cumulative emissions of carbon dioxide set the amount of warming. The warming caused by CO₂ in recent decades is, to a good approximation, just linearly proportional to the total cumulative amount of carbon emitted into the atmosphere. We do not know exactly how much added CO2 will produce how much warming—that is, the climate sensitivity question—but we can estimate a range of possible answers to this question, constrained by different kinds of observations. For the middle of the range, 1 trillion metric tons of carbon emitted produces a warming of about 2 degrees Celsius above the temperatures of the early 1800s. We have already emitted about half of this amount. At present, the warming we observe is caused by CO₂ plus several other heat-trapping substances that human activities have also added to the atmosphere. The amounts in the atmosphere of these other substances will decrease rapidly when and if their sources are eliminated, but some of the carbon dioxide will remain in the atmosphere for thousands of years. Because of the difference in the amount of time that different heat-trapping substances stay in the atmosphere, carbon dioxide is truly the key “control knob” for climate.

Reducing emissions of carbon dioxide and other heat-trapping substances will limit the warming. We can estimate the cumulative amount of carbon emitted that would give us a good chance of limiting warming to 2 degrees Celsius (that’s 3.6 degrees Fahrenheit) above the pre-industrial temperatures of the early 1800s. This is the warming target endorsed by the Paris Agreement of 2015. If emissions had peaked and began to decline several decades ago, then emissions reductions could be gradual, and by 2050 emissions would not yet need to have entirely stopped. Because emissions are still increasing, drastic emissions reductions need to occur quickly and reach zero by 2040.

“Negative emissions,” meaning removing some carbon dioxide from the air, are likely to be necessary. This fact illustrates the urgency of acting. Finding a way of removing some of the carbon dioxide is one approach to geoengineering. Here by “geoengineering” we mean the intentional modification of the climate system with the goal of reducing or mitigating climate change. However, nobody has yet demonstrated a way of economically removing large amounts of carbon dioxide from the atmosphere.

The longer we wait before acting, the more drastic the action has to be. The result of failing to act is to increase the likelihood of dangerous climate change.

Because it takes so long for the carbon dioxide in the atmosphere to decrease, climate change will last for centuries. After emissions completely stop, the amount of carbon dioxide in the atmosphere decreases only slowly for several centuries, and about 25 percent of it remains in the atmosphere for the next 10,000 years or so. The science relevant to this topic is not simple. Several complex processes for carbon removal are involved. The key take-away message is that the climate change caused by adding carbon dioxide to the atmosphere can have very long-lasting effects.

The conclusions I have just recounted are facts. They are fundamental findings from extensive scientific research. They are all well-supported by abundant evidence.

The science is never complete. There is always more to learn. But the science that we have now is already good enough to help us make wise decisions. The many unknowns in the science, such as exactly how fast the Antarctic ice sheet will shrink or exactly how El Niño might be affected, are not the biggest unknowns about future climate. The biggest unknown about future climate is human behavior. Everything depends on what people and their governments do.

The scientific consensus is overwhelming. Climate change is already happening, here and now. About 97 percent of climate experts—the scientists who are most active in carrying out and publishing research on climate change—agree that the observed recent warming is real and serious and overwhelmingly human-caused, and that it will become even more serious unless we make big changes in how we generate energy. Nevertheless, some people remain unconvinced. They continue to repeat climate myths and falsehoods.

People often ask me, “I’m only one person. What can I do about climate change?” Here is my answer. We need to persuade more people that this problem is serious. Governments tend to respond when enough people become concerned, and when they vote their concerns. I urge everyone to engage with people you may know—family, friends, colleagues—who don’t accept the fundamental findings of climate science. Explain to them the facts you have learned about our changing climate. Listen to them respectfully and carefully. Be alert to the common climate myths and falsehoods that they may think are true. If you see something, say something. Have a civil conversation. Have many conversations. In their hearts, almost all of us would surely agree that everybody is entitled to his own opinions, but not to his own facts. And it is science that supplies the facts about climate change.

We humans have become the dominant actors in causing the rapid climate change we now observe. Human actions now overwhelm all the natural processes. This may seem counter-intuitive, but it’s true. You and I, and all the people who are alive today, now have our hands on the thermostat that controls the climate of our children and grandchildren. Metaphors can be superb communication tools. The thermostat is a powerful metaphor.

Think about medical metaphors. Here are a few: We climate scientists are planetary physicians. Climate science and medical science will both always be imperfect and incomplete, but both are already very useful.

When your doctor tells you to stop smoking and lose weight and exercise more, you don’t argue with her. You don’t call her a radical alarmist. You don’t ask her to name the date when you will have a heart attack.

Physicians have advanced academic credentials and many years of training and experience. We climate scientists have the same. We’re not conspiring to fool people. Do you really think your doctor is a crook? She’s not. Neither are we.

A fever of only a few degrees can indicate a serious disease. Global warming is just a symptom of planetary ill health, like a fever.

Prevention is better than cure. Quitting smoking, like quitting using fossil fuels, is not easy to do. And the main benefits of quitting come in the long-term future.

Choosing to have major surgery involves cost and risk. People know that choosing to do nothing also has costs and risks.

The laws of climate science and medical science are all immune from political tampering. You can’t fool Mother Nature. Mother Nature always bats last.

Here’s an effective metaphor. Imagine you are watching a major-league baseball game. The slugger who is thought to be on performance-enhancing drugs hits a home run. The person next to you asks, did the steroids cause it? That’s really the wrong question. You can’t be sure they caused it, because he was already a big-league slugger when he was clean. And even with the drugs, he can still strike out now and then. But at the end of the season, you see in his statistics that he hit more homers than he used to. The steroids increase the odds of home runs. Climate is the statistics of weather, and carbon dioxide is the steroids of climate.[2] It changes the odds. The odds are higher now for all sorts of extreme weather, because climate change has altered the environment in which all weather occurs.

This metaphor works for other sports too. For example, baseball isn’t popular in France, but bicycle racing is very popular there, and French people know that a bike racer on drugs won’t win every race, but the drugs do change the odds and increase his probability of winning.

The main barriers to action on climate change are a lack of widespread political will and a lack of wise and inspiring leadership. Science can help to inform policy, but only concerned people and responsive, capable governments can first decide what policies are best, and then implement them. Today, despite a strong scientific consensus, climate change is controversial politically.

We do not have to accept a future with devastating climate damage and disruption. If we continue to use more and more fossil fuels to generate the world’s energy, we will be sentencing our children and grandchildren to many centuries with a severely damaged climate and great suffering. In your conversations, try to help people understand that this bleak future is entirely preventable.

Faced with these threats, almost all the nations of the world agreed in Paris in late 2015 to limit the warming to a specific maximum amount. That amount is 2 degrees Celsius, or 3.6 degrees Fahrenheit, above the average global temperature in the early 1800s, before human activities began to have a large effect

After Paris, is the glass half-empty or half-full? I am guardedly optimistic, for these reasons:

• World leaders are now engaged; at least almost all of them are.
• Emissions of heat-trapping gases have begun to decline in some places.
• Solar and wind energy are getting cheaper every year.
• Renewable energy use is increasing rapidly.
• Many corporations are now acting to reduce emissions.
• State and local governments in the United States are acting too, despite federal inaction.
• Many other countries are showing rapid progress.

Recent polling shows that in the United States, many more people accept the science and are very concerned about global warming or climate change than was the case only a few years ago. However, the issue has become extremely partisan. Recent polls show that the substantial increase in the number of Americans calling climate change a top priority has been limited to the Democratic side of the political spectrum. However, about 80 percent of Republicans, including virtually the entire leadership of the Republican party, have not changed their minds and still reject the science. We have a long way to go. I think we should keep climate change science separated from climate change policy.

There is no silver bullet that solves all the challenges of climate change, but there is a lot of silver buckshot, including increased energy efficiency and energy conservation, and much more use of sun, wind and water to provide the energy the world needs. These renewable resources are widely available now and already cost-competitive with fossil fuels. We have the technology, and it is improving. In the United States, even without energetic action by the federal government, I am guardedly optimistic.

Market forces now favor carbon-free energy. Coal companies are going bankrupt. Solar and wind energy without subsidies are in many cases already cheaper than fossil fuels. Electric vehicles are happening fast. Much energy policy in the US is set at state and local levels, not in Washington.

Always remember why we want to have conversations about climate change science. We want to inform people. We want to motivate them. We want them to act.

Research suggests that messages that may invoke fear or dismay are better received if they also include hope. We should include positive messages about our ability to solve the problem. We can explain that future climate is in our hands.

Politics and priorities and values do have a role to play in deciding which actions are best, but any rational policy begins by accepting the science. People are entitled to their own opinions, but not to their own facts.

This article is being written in 2020 during the global coronavirus pandemic, and I have a few gentle words to say about some climate change lessons that we might learn from the pandemic now gripping the entire world.

One obvious point is that climate change science, like coronavirus epidemiology, is incomplete, still developing but already extremely useful. In both domains, we have learned we can trust scientists more than politicians or pundits or anybody else who is not really an expert on the science of the subject, whether the subject is climate change or infectious diseases. We have also learned that the challenges in both climate change and the pandemic are global. The entire world is affected. The solutions have to be global too.

The pandemic also illustrates the wisdom of the statement that, “Everybody is entitled to his own opinions, but not to his own facts.” The facts about climate change, and about COVID-19, are objective truth, and they should be the same for everybody, regardless of people’s ideology or politics. When it comes to making policy, sound science can inform wise policy. However, policy can also depend on many other factors, such as people’s priorities, their convictions about economics, what they regard as the proper role of governments, their risk tolerance, and, of course, public opinion. That’s true for meeting the challenge of climate change, just as for meeting the challenge of COVID-19.

The pandemic reminds us how valuable science and scientists are. The recent discussions in the news—such as about how clinical trials of drugs and vaccines work—are very educational. The medical scientists who develop new medicines do their best to make sure they are safe and effective, and they won’t release them for widespread use to the public until they are absolutely convinced of that. They are real experts and are very careful.

So are climate scientists.

Tony Fauci, who has often been on television recently in the United States, is a good example of a person who is a real expert on pandemics. He says simply, “I’m a physician and a scientist.” People get that. When Fauci speaks to the public, he is not trying to be popular or make people happy or brag about himself or make money. He is just telling people the facts that scientists have discovered, and he describes these facts in a way that is honest and transparent and understandable. He has spent his entire career accumulating expertise and experience and wisdom about infectious diseases and pandemics. People should trust him, and polls show that the great majority of people do trust him. Science is absolutely essential. That’s true for pandemics.

It’s also true for climate change.

Like many interested people, I watched Warren Buffett’s 2020 Berkshire Hathaway stockholders meeting online. Warren Buffett, the brilliant investor and one of the richest people in the world, agrees with me. Here’s what he said:

“I think, personally I feel extraordinarily good about being able to listen to Dr. Fauci, who I had never heard of a year ago. But I think we’re very, very fortunate as a country to have somebody at 79 years of age who appears to be able to work 24 hours a day and keep a good humor about him and communicate in a very, very straightforward matter about fairly complex subjects and tell you when he knows something and when he doesn’t know something. So, I’m not going to talk about any political figures at all or our politics generally this afternoon, but I do feel that I owe a huge debt of gratitude to Dr. Fauci for educating and informing me, actually along with my friend Bill Gates, too, as to what’s going on. I know I get it from a straight shooter when I get it from either one of those. So, thank you Dr. Fauci.”