Climate Science Summary
Last Update: 10.09.15
Hardly a day goes by without a news item appearing about a recent development involving climate change or global warming. If you have not followed this topic closely you may wonder exactly what these terms mean. More importantly, you may wonder why it is occurring and what aspects of climate change are climate scientists in virtually unanimous agreement about. Or, where is there still substantial uncertainty?
This summary is intended to help answer these questions and to direct those who want further information to other entries on this website, especially the lessons on the Tutorial page. In the Tutorial all the topics below are discussed in more detail and references and figures and graphs illustrating them can be found. There are also a number of erroneous objections to the basic science, which are addressed by the Misperceptions page.
This summary is intended to help answer these questions and to direct those who want further information to other entries on this website, especially the lessons on the Tutorial page. In the Tutorial all the topics below are discussed in more detail and references and figures and graphs illustrating them can be found. There are also a number of erroneous objections to the basic science, which are addressed by the Misperceptions page.
The Basic Science
First, it is important to distinguish between climate and weather. In any specific location climate refers to the long-term average of the usual things that are reported in daily weather reports. In Atascadero, California, where I live, we expect warm, dry summers and mild winters with (normally) some rain and occasional frosts. But of course each summer day is not the same—we have heat waves followed by milder days in summer and in winter we experience rainstorms with periods of sun between. These day-to-day changes constitute weather. In order to establish whether the climate is changing, we need to look at long-term data (at least 20 years) since weather patterns may come and go--not just from day to day but also from year to year. In fact, the changing weather patterns we associate with "El Niño and La Niña" come and go over periods of a few years.
We can also examine data averaged over the entire world to see if, for example, the globally averaged air temperature or the ocean temperature or the total amounts of precipitation show long-term trends. In fact, they do. However, superimposed on such trends are variable weather patterns (called by climate scientists “natural internal variability”) so that not every year is inexorably warmer than the preceding year. The trend of increasing globally averaged atmospheric and ocean surface temperatures, especially over the last four decades, is now unmistakable and this is what “global warming” refers to.
Accompanying this warming are changes in the Earth’s climate system both locally and globally that are equally unmistakable when examined over similar times scales. Among these are: rising sea level, loss of sea ice in the arctic, loss of ice from the Antarctic and Greenland ice sheets, and increasing frequency and severity of heat waves. An additional change not directly related to global warming but caused by the agent now responsible for the present warming (see below) is the dramatic change in the chemistry of the ocean (“ocean acidification”).
What causes the climate to change? Fundamentally, climate changes when there is a persistent imbalance, even though small, in the amount of heat energy entering or leaving the Earth’s climate system. By the Earth’s climate system I mean the land surface, the atmosphere, the oceans and the “cryosphere”—all frozen water on the planet: the glaciers, sea ice and the two enormous ice sheets on Antarctica and Greenland. That such an imbalance has been occurring is incontrovertible: It has been measured directly by satellites, but the warming atmosphere and especially the warming oceans also directly measure this imbalance. One of the most fundamental laws in all of physics is that an increase in energy in a system like the Earth’s climate system demands that there be a flow of energy into it.
The following statements are also so well established as to be considered absolutely certain:
1) Over the last 200 years or so, and especially during the last 50, the composition of certain molecules in the atmosphere crucial to regulating the flow of energy, especially carbon dioxide, have spiked dramatically.
2) This spike is due essentially 100% to human activities, including deforestation but mostly due to combustion of coal, oil and natural gas (“fossil fuels”) one of whose combustion by-products is carbon dioxide.
3) This increase in carbon dioxide (now at levels that have not occurred naturally in at least three million years) has resulted in increased trapping of the infrared radiation that the Earth radiates into space. Except for the last century or so this outgoing infrared heat flow balanced the incoming heat flow from the sun. The basic mechanism of this trapping—the “greenhouse gas effect” has been understood for over a century and has been thoroughly observed not only in the Earth’s atmosphere but the same basic laws governing how heat and light energy moves through a gaseous atmosphere find precise confirmation in observations of stars as well.
I do not know of a single climate scientist who honestly disagrees with any of these three statements.
So where does any disagreement arise? In addition to the direct warming caused by increased heat trapping that in turn is caused by increased greenhouse gases, there are indirect effects associated with greenhouse gas warming. These are discussed in Tutorial Lesson # 2: Forcings and Feedbacks. “Positive feedbacks” amplify the warming caused by the increase in greenhouse gases. The most important of these involves water vapor: As the ocean and atmosphere warm, more water vapor is able to be held in the atmosphere without falling back as rain. Since water vapor itself is a potent blocker of outgoing infrared heat, this increases the amount of heat being trapped and enhances the amount of warming due to human-produced greenhouse gases.
A minority of climate scientists—(a very tiny minority; some surveys suggest one in 30; others less than one in 100) disagree that the present warming is mostly due to carbon dioxide. Some of these “climate contrarians” argue that there are possible “negative feedbacks” offsetting the greenhouse gas heating and/or that the positive feedbacks have been over-estimated. They then argue that the rise in atmospheric temperature is due to chance “natural internal variability”. The vast majority of climate scientists reject this hypothesis. Furthermore, it is even much harder to see what can be responsible for the very much larger increase in the heat content of the oceans which is much larger than that in the atmosphere.
Other “climate contrarians” maintain that changes in the sun are responsible for the recent warming. This, despite the fact that extremely accurate satellite observations show no increase in the sun’s energy output averaged over the last 40 years. Moreover there are distinct observed “fingerprints” in the climate system that are what is expected from increased greenhouse gas warming but are contrary to what would be expected if the warming were due to the sun.
In summary, the overwhelming consensus among research climate scientists, based upon the laws of physics, along with the observations I have outlined above, is that the warming we are experiencing is attributable to human activities, especially the burning of fossil fuels.
Does this then mean “climate science is settled science?” The phrase “settled science” is often used (or rather misused) by those who do not understand how science and scientists work. Consensus gradually emerges over some aspects of any scientific discipline. For example, the basic idea of the evolution of biological species due to a combination of genetic mutation and natural selection is “settled science” but disagreements abound in some of the details of how this plays out. (See, for example, the book by E.O. Wilson: “The Meaning of Human Existence”.) Similarly, the basic ideas outlined above about climate science can be considered “settled” but there is vigorous debate on the details of how this plays out in the complexities of the Earth’s climate. For example, it is well established that the arctic regions are warming at a much faster rate than the tropics and subtropics and there is a group of scientists who think that this is altering the characteristics of the jet stream, resulting, in turn, in more persistent weather patterns and possibly diverting storm tracks from their usual course. Others think this is far from proven, and so this remains an area of vigorous research and discussion. So, when someone utters the remark that “climate science is not settled science” you can be fairly certain that person does not understand how science progresses.
We can also examine data averaged over the entire world to see if, for example, the globally averaged air temperature or the ocean temperature or the total amounts of precipitation show long-term trends. In fact, they do. However, superimposed on such trends are variable weather patterns (called by climate scientists “natural internal variability”) so that not every year is inexorably warmer than the preceding year. The trend of increasing globally averaged atmospheric and ocean surface temperatures, especially over the last four decades, is now unmistakable and this is what “global warming” refers to.
Accompanying this warming are changes in the Earth’s climate system both locally and globally that are equally unmistakable when examined over similar times scales. Among these are: rising sea level, loss of sea ice in the arctic, loss of ice from the Antarctic and Greenland ice sheets, and increasing frequency and severity of heat waves. An additional change not directly related to global warming but caused by the agent now responsible for the present warming (see below) is the dramatic change in the chemistry of the ocean (“ocean acidification”).
What causes the climate to change? Fundamentally, climate changes when there is a persistent imbalance, even though small, in the amount of heat energy entering or leaving the Earth’s climate system. By the Earth’s climate system I mean the land surface, the atmosphere, the oceans and the “cryosphere”—all frozen water on the planet: the glaciers, sea ice and the two enormous ice sheets on Antarctica and Greenland. That such an imbalance has been occurring is incontrovertible: It has been measured directly by satellites, but the warming atmosphere and especially the warming oceans also directly measure this imbalance. One of the most fundamental laws in all of physics is that an increase in energy in a system like the Earth’s climate system demands that there be a flow of energy into it.
The following statements are also so well established as to be considered absolutely certain:
1) Over the last 200 years or so, and especially during the last 50, the composition of certain molecules in the atmosphere crucial to regulating the flow of energy, especially carbon dioxide, have spiked dramatically.
2) This spike is due essentially 100% to human activities, including deforestation but mostly due to combustion of coal, oil and natural gas (“fossil fuels”) one of whose combustion by-products is carbon dioxide.
3) This increase in carbon dioxide (now at levels that have not occurred naturally in at least three million years) has resulted in increased trapping of the infrared radiation that the Earth radiates into space. Except for the last century or so this outgoing infrared heat flow balanced the incoming heat flow from the sun. The basic mechanism of this trapping—the “greenhouse gas effect” has been understood for over a century and has been thoroughly observed not only in the Earth’s atmosphere but the same basic laws governing how heat and light energy moves through a gaseous atmosphere find precise confirmation in observations of stars as well.
I do not know of a single climate scientist who honestly disagrees with any of these three statements.
So where does any disagreement arise? In addition to the direct warming caused by increased heat trapping that in turn is caused by increased greenhouse gases, there are indirect effects associated with greenhouse gas warming. These are discussed in Tutorial Lesson # 2: Forcings and Feedbacks. “Positive feedbacks” amplify the warming caused by the increase in greenhouse gases. The most important of these involves water vapor: As the ocean and atmosphere warm, more water vapor is able to be held in the atmosphere without falling back as rain. Since water vapor itself is a potent blocker of outgoing infrared heat, this increases the amount of heat being trapped and enhances the amount of warming due to human-produced greenhouse gases.
A minority of climate scientists—(a very tiny minority; some surveys suggest one in 30; others less than one in 100) disagree that the present warming is mostly due to carbon dioxide. Some of these “climate contrarians” argue that there are possible “negative feedbacks” offsetting the greenhouse gas heating and/or that the positive feedbacks have been over-estimated. They then argue that the rise in atmospheric temperature is due to chance “natural internal variability”. The vast majority of climate scientists reject this hypothesis. Furthermore, it is even much harder to see what can be responsible for the very much larger increase in the heat content of the oceans which is much larger than that in the atmosphere.
Other “climate contrarians” maintain that changes in the sun are responsible for the recent warming. This, despite the fact that extremely accurate satellite observations show no increase in the sun’s energy output averaged over the last 40 years. Moreover there are distinct observed “fingerprints” in the climate system that are what is expected from increased greenhouse gas warming but are contrary to what would be expected if the warming were due to the sun.
In summary, the overwhelming consensus among research climate scientists, based upon the laws of physics, along with the observations I have outlined above, is that the warming we are experiencing is attributable to human activities, especially the burning of fossil fuels.
Does this then mean “climate science is settled science?” The phrase “settled science” is often used (or rather misused) by those who do not understand how science and scientists work. Consensus gradually emerges over some aspects of any scientific discipline. For example, the basic idea of the evolution of biological species due to a combination of genetic mutation and natural selection is “settled science” but disagreements abound in some of the details of how this plays out. (See, for example, the book by E.O. Wilson: “The Meaning of Human Existence”.) Similarly, the basic ideas outlined above about climate science can be considered “settled” but there is vigorous debate on the details of how this plays out in the complexities of the Earth’s climate. For example, it is well established that the arctic regions are warming at a much faster rate than the tropics and subtropics and there is a group of scientists who think that this is altering the characteristics of the jet stream, resulting, in turn, in more persistent weather patterns and possibly diverting storm tracks from their usual course. Others think this is far from proven, and so this remains an area of vigorous research and discussion. So, when someone utters the remark that “climate science is not settled science” you can be fairly certain that person does not understand how science progresses.
Impacts, Adaptation and Mitigation
Even granting the basic facts of human-induced warming, there are those who, for a variety of reasons, oppose sharply limiting the emissions from fossil fuels and switching to non-emitting sources of energy. Their objections usually take one of the following three forms:
1) Additional warming will do no harm and may be good for us.
2) There are still too many uncertainties in understanding our climate: let’s delay action until we are sure.
3) It will be too expensive and damaging to the economy to switch from fossil fuels to alternative sources of energy.
Here are brief responses to these three objections:
Regarding objection (1) it is true that some consequences of climate change and increasing carbon dioxide will be beneficial. Growing seasons in some northern latitudes will be increased and some crop yields are increased with increased carbon dioxide. But this latter effect is more than offset by increasing heat waves, drought and flooding, and food supply will be adversely affected, especially in those countries least able to cope with these changes. Moreover, impacts such as rising sea levels and associated storm surges will be costly and displace hundreds of millions. The costs of more severe weather and rising sea level are already recognized by much of the insurance industry. More severe and frequent heat waves and the spread of insects carrying diseases will have severe health impacts. Wildfires will become more severe, destroying timber and property as the western U.S. has sadly experienced in 2015. Water shortages will become more severe causing social unrest, an issue already recognized by our military and intelligence community.
Regarding objection (2) the scientific case for reducing the use of fossil fuel and minimizing the risks of irreversible changes (“tipping points”) in aspects of the climate are well enough established to warrant prompt action. Furthermore, the “let’s wait” suggestion ignores the important point that the buildup of greenhouse gases humans are causing will take hundreds of years to be restored to their pre-industrial levels by natural processes even if emissions were to cease immediately. During this time, continued warming will occur. Simulations of the effects of increased greenhouse gases on future warming show that the maximum global temperature experienced depends upon the total cumulative amount of emissions. The longer we defer action to curb emissions the closer we come to exceeding a “budget” limiting the amount of warming to some maximum temperature target. Thus, the longer we postpone action, the narrower the window of time and the more difficult it becomes not to exceed this budget. Every additional degree of increase in the maximum temperature reached as global warming occurs strongly increases the severity of the impacts of the resultant changes in climate and increases the likelihood that irreversible tipping points (for example, loss of the Brazilian rain forest and the disintegration of the West Antarctic Ice sheet) will occur.
Finally, regarding objection (3) and the economic costs of switching from fossil fuel to alternative energy sources, numerous studies of the economic trade-offs between “business as usual” practices versus large reductions in fossil fuel use show that the costs of inaction far exceed the cost of switching to alternative fuels. Typical estimates of the latter involve annual expenditures of about 1 % of the world’s Gross Domestic Product (GDP).
For a detailed example of a cost/benefit analysis of dealing with climate change by a leading “climate economist”, see “The Climate Casino” by William Nordhaus”. However, in my opinion, his optimum strategy is too cautious given the continual rapid decrease in cost and novel innovations for alternative energy sources along with recent realizations that we are dangerously close to passing irreversible tipping points in the climate system. Additionally, as explained in my essay on this website (Essay # 8: Why Should California Bother), switching to a low carbon economy has already provided a stimulus to the economy and provides a strong incentive for further technological innovations to bring down the costs of renewable energy lower still.
1) Additional warming will do no harm and may be good for us.
2) There are still too many uncertainties in understanding our climate: let’s delay action until we are sure.
3) It will be too expensive and damaging to the economy to switch from fossil fuels to alternative sources of energy.
Here are brief responses to these three objections:
Regarding objection (1) it is true that some consequences of climate change and increasing carbon dioxide will be beneficial. Growing seasons in some northern latitudes will be increased and some crop yields are increased with increased carbon dioxide. But this latter effect is more than offset by increasing heat waves, drought and flooding, and food supply will be adversely affected, especially in those countries least able to cope with these changes. Moreover, impacts such as rising sea levels and associated storm surges will be costly and displace hundreds of millions. The costs of more severe weather and rising sea level are already recognized by much of the insurance industry. More severe and frequent heat waves and the spread of insects carrying diseases will have severe health impacts. Wildfires will become more severe, destroying timber and property as the western U.S. has sadly experienced in 2015. Water shortages will become more severe causing social unrest, an issue already recognized by our military and intelligence community.
Regarding objection (2) the scientific case for reducing the use of fossil fuel and minimizing the risks of irreversible changes (“tipping points”) in aspects of the climate are well enough established to warrant prompt action. Furthermore, the “let’s wait” suggestion ignores the important point that the buildup of greenhouse gases humans are causing will take hundreds of years to be restored to their pre-industrial levels by natural processes even if emissions were to cease immediately. During this time, continued warming will occur. Simulations of the effects of increased greenhouse gases on future warming show that the maximum global temperature experienced depends upon the total cumulative amount of emissions. The longer we defer action to curb emissions the closer we come to exceeding a “budget” limiting the amount of warming to some maximum temperature target. Thus, the longer we postpone action, the narrower the window of time and the more difficult it becomes not to exceed this budget. Every additional degree of increase in the maximum temperature reached as global warming occurs strongly increases the severity of the impacts of the resultant changes in climate and increases the likelihood that irreversible tipping points (for example, loss of the Brazilian rain forest and the disintegration of the West Antarctic Ice sheet) will occur.
Finally, regarding objection (3) and the economic costs of switching from fossil fuel to alternative energy sources, numerous studies of the economic trade-offs between “business as usual” practices versus large reductions in fossil fuel use show that the costs of inaction far exceed the cost of switching to alternative fuels. Typical estimates of the latter involve annual expenditures of about 1 % of the world’s Gross Domestic Product (GDP).
For a detailed example of a cost/benefit analysis of dealing with climate change by a leading “climate economist”, see “The Climate Casino” by William Nordhaus”. However, in my opinion, his optimum strategy is too cautious given the continual rapid decrease in cost and novel innovations for alternative energy sources along with recent realizations that we are dangerously close to passing irreversible tipping points in the climate system. Additionally, as explained in my essay on this website (Essay # 8: Why Should California Bother), switching to a low carbon economy has already provided a stimulus to the economy and provides a strong incentive for further technological innovations to bring down the costs of renewable energy lower still.