Global Warming

Given all the back and forth on climate change out there, it can be hard to know what to think. It's also a complicated subject, so much of the information is distributed all over the place or in huge tomes. Below I've put together a fairly quick overview that I've used for teaching, which might be helpful to others.

One of the key things to keep in mind is that there are actually several distinct questions surrounding global warming, even though they're often conflated into a single 'climate debate.' Some we know the answers to very well, others less so. Here's the progression of questions, as I see them:

1. Is the climate changing?
2. Are humans responsible?
3. How do we know the CO2 is ours?
4. How much will climate change in the future?
5. How big of a deal is this?
6. What, if anything, can we do about it?

1. Is the climate changing?
Yes, the evidence for global warming over the past century is ‘unequivocal’, and comes from:
i. Worldwide thermometer records (from rural and urban locations)
ii. Satellites
iii. The retreat of most (though not all) mountain glaciers
iv. Rising sea level
v. Increasing ocean heat content
And two other pieces of evidence, though not global in scale:
vi. Declining Arctic summer sea ice
vii. Ice loss from Greenland and Antarctica

2. Are humans responsible?
Yes, it is ‘extremely likely’ that anthropogenic greenhouse gases have caused most of the global warming since 1950. Here are several of the so-called ‘proofs’ of human-caused global warming:
i. Three main knobs control the global thermostat: (1) the sun, (2) the earth’s reflectivity, and (3) the strength of earth’s greenhouse effect, and only (3) explains recent warming. The is easy to see – the sun’s output has been essentially flat over the 40-year satellite record (no change), earth’s reflectivity has probably increased due largely to human land-use changes and pollution (cooling), and the greenhouse effect has gotten considerably stronger due to human greenhouse gas emissions (warming). Incidentally, CO2’s warming tendency is long-established physics that was first discovered in 1859, and ice core records show that CO2 levels today are far higher than any time in at least the past 800,000 years.
ii. The vertical fingerprint of a warming lower atmosphere and cooling upper atmosphere is what is expected from CO2-driven climate change (e.g., as opposed to the sun, which would warm the whole atmosphere).
iii. So are the horizontal fingerprints of near-worldwide warming, greater warming over land than ocean, and fastest warming in the Arctic.
iv. Satellites have observed the increased trapping of radiation in the atmosphere at the wavelengths that greenhouse gases absorb.
v. Ocean heat content has increased over the past 50 years. This means that global surface warming can’t just be due to the ocean coughing up heat to the surface, and also that CO2 has more than overpowered the recent deep solar minimum.
vi. Climate models (both dynamical and statistical) can’t explain recent warming with natural factors (solar, volcanic) alone, but have to include greenhouse gases.
vii. Proxy temperature records from e.g., tree rings, corals, ice cores, etc. show that recent warmth is unprecedented in at least 1000 years, pointing to something different/unnatural going on today. This result has been independently confirmed by multiple methods, datasets, and teams of scientists.

It’s worth highlighting that the general consensus on human-caused global warming grew slowly over the years as the evidence strengthened – as it should in any good, cautious science. For example, note how the main conclusion of the UN Intergovernmental Panel on Climate Change, which issues synthesis reports every half decade or so, has progressed:
1990: The unequivocal detection of the enhanced greenhouse effect is not likely for a decade or more
1995: The balance of evidence suggests a discernible human influence on global climate
2001: There is new and stronger evidence that most of the warming over the last 50 years is attributable to human activities
2007: Most of the observed increase in global average temperature since the mid-20th century is very likely due to observed increase in anthropogenic greenhouse gas concentrations
2013: It is extremely likely that more than half of the observed increase in global average surface temperature from 1951-2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together

3. How do we know the CO2 is ours?
It’s rarely even questioned that rising CO2 comes mostly from us burning fossil fuels, but it’s good to know the basis:
i. Fossil fuels are a globally traded commodity and people count their coins, so we have a reasonable accounting of our fossil fuel consumption. We’ve actually emitted twice as much CO2 as explains the recent rise in the atmosphere, but about half of our CO2 has been absorbed by the ocean (acidifying it) and terrestrial biosphere (fertilizing it).
ii. The radiocarbon content of atmospheric CO2 has been decreasing, which means that the source is very old (e.g., fossil fuels).
iii. The carbon-13 content of atmospheric CO2 has also been decreasing, which is consistent with a biological source (e.g., fossil fuels) since living things have relatively little carbon-13 in them.
iv. Oxygen levels in the atmosphere have been declining as CO2 levels have been rising, which points to the CO2 coming from combustion (e.g., of fossil fuels).
v. CO2 concentrations have risen slightly faster in the Northern Hemipshere, indicating the main source (e.g., most people and industrialization) is there.
vi. CO2 is going into the ocean and decreasing its pH (i.e., CO2 is not coming from the ocean).
vii. Humans are emitting CO2 ~50 times faster than volcanoes.
viii. Atmospheric CO2 levels were nearly constant for the 10,000 years prior to the Industrial Revolution indicating natural carbon sources and sinks were in balance before we stepped on the scene.
ix. The rate of atmospheric CO2 increase has tracked the rate of human emissions, both increasing significantly over the past century.

4. How much will climate change in the future?
This depends on two main things: (i) how much CO2 we emit, and (2) how sensitive the climate is to CO2.
i. CO2 emissions are a social science question and obviously depend on policy, economics, population, etc. This is often reduced to a simple equation for global emissions (the so-called Kaya identity): population X wealth ($ per person) X energy intensity (energy used per $) X carbon intensity (carbon emitted per unit energy). These are the four knobs controlling emissions, there are no others.Depending on what trajectory society takes, estimates are that we will probably double (on the low end) to triple or quadruple (on the high end) atmospheric CO2 levels over this century from where they were in preindustrial times.
ii. Climate sensitivity to CO2 is a physical science question, and is usually talked about in terms of how much global warming you get for a doubling of CO2. The warming due to doubling CO2 alone is straight physics and easy to calculate – 1.2°C. All of the uncertainty in global warming projections comes from knowing how everything else in the climate will respond to this initial warming and either amplify or dampen it. These are climate feedbacks and include things like clouds, water vapor, and snow cover. The best estimate, based on lots of different evidence, is that feedbacks in total amplify the initial CO2 warming, anywhere from a little to a lot, so that the final warming from doubling CO2 is maybe 1.5-4.5°C.
The bottom line is that climate will continue to warm as we emit more CO2, and the more we emit, the more it warms. Exactly how much isn’t quite clear, though for a low-emission scenario perhaps just another 1°C, while a high-emission scenario would likely mean several more degrees.

5. How big of a deal is this?
The questions above are interesting to think about and important to scientists, but this is the one that really matters to us all. I don’t think anyone has a precise answer to this question, and it strikes me as somewhat similar to economists trying to answer what exactly would happen if the US defaulted on its debt – as with the economists and the default question, I think most climate scientists have a pretty uneasy feeling about where the world goes with unchecked global warming, but they can’t map out precisely what it will look like. Beyond the sizeable range in how sensitive global temperature is to CO2 mentioned above, there are some big questions over if or when it will lead us across tipping points such as the loss of Arctic sea ice, rapid ice sheet changes and sea level rise, or methane burps from melting permafrost. A few things that are clear in answering this question though is that it depends on how much carbon we emit, the pace of climate change (e.g., abrupt jumps versus gradual trends), how well we are able to adapt (e.g., to sea level rise, impacts on agriculture, etc.), and what decade/generation you’re thinking about (e.g., the baby boomers will probably not be too affected, the millennials and their kids and grandkids may be a different story). One other catch in this last regard is that, unlike many other issues we’re used to dealing with, there is considerable inertia with climate change, and so we’re almost guaranteed to overshoot the mark. For instance, CO2 levels will remain elevated for centuries to millennia even after we zero emissions, so climate won’t just start cooling back down then; in fact, the climate is continually playing catch-up with our CO2 because we have big oceans that take a long time to heat up, and so there is already more ‘warming in the pipeline’, even if we stabilized CO2 levels today. Likewise, the Greenland and Antarctic Ice Sheets take a long time to come into balance with a new climate state, so they will continue melting for centuries or more and raise sea level. And a final source of inertia is the time it would take to switch the global energy infrastructure over to non-carbon emitting sources of energy if we decided to go more in that direction. 

6. What, if anything, can we do about it?
There are a few possibilities here:
i. Adapt. We are already committed to some more climate change as mentioned above, so some degree of adaptation is a given, but insofar as we don’t try to mitigate climate change, this is the main card we’ll play. An important, not-so-easy to answer question is exactly how much this would cost... in terms of GDP (reduced manual labor in summer, insurance premiums, more Sandy's and Katrina's), societal stability (food/water security, flows of refugees, inlanders tax dollars to rebuild/fortify coastal cities), and things it's hard to attach a dollar value to (e.g., in New England, maybe skiing, vibrant fall foliage, maple sugaring, a white Christmas...).
ii. Mitigate. This means reducing carbon emissions by improving efficiency and switching away from fossil fuels. Given that most alternative energies are costlier than fossil fuels these days, it’s hard to see this happening in a big way without policy changes (e.g., a carbon tax), and that gets tricky…
iii. Geoengineering. There are two main ideas usually discussed on this. One is to actively suck CO2 out of the air – this is much too expensive to be particularly relevant at this point. The other is to inject sulfur aerosols into the atmosphere with a hose to block out sunlight and cool the planet, offsetting greenhouse warming. This happens naturally for a year or two after big volcanic eruptions and it is quite cheap, so we know it could work and it is logistically feasible. There are unfortunately drawbacks to this, some potentially quite big. Sunlight drives photosynthesis, and so life on the planet, and it’s not clear how blocking sunlight globally over the long haul would affect biology. This also doesn’t address another big issue related to carbon emissions – ocean acidification. Furthermore, despite stabilizing global mean temperature, there would likely be other effects on climate from dialing down solar energy, such as changing rainfall patterns. An additional complication is that aerosols wash out of the atmosphere in a year or so, while CO2 stays up there for centuries, so this geoengineering would have to go on indefinitely and ramp up in intensity to continue counteracting the ever-increasing buildup of greenhouse gases; in this sense, geoengineering is maybe more accurately viewed as a tourniquet than a bandaid. And if for some reason the pump stopped (world war, terrorist attack, etc.), there would be an abrupt global warming event as the aerosols washed out of the air and the previously-masked greenhouse effect took over, which could be much tougher to deal with than the gradual global warming that would have occurred without geoengineering in the first place. Lastly, the most basic issue of all would be politically working out who would control the global thermostat, which is fraught with conflicting interests among various nations.


Common Misconceptions

Global warming caused Hurricane Sandy
Climate is the statistics of weather, and climate change is a shift in the statistics of weather. So, obviously no single weather event can ever really be conclusively pinned on climate change. At the same time, this doesn’t mean climate change has no effect on individual events. A more useful way to think about it is how much climate change affects the likelihood of an event happening (‘loading the dice’) or perhaps contributes to its intensity. Some analogies are helpful here. Barry Bonds would have hit plenty of home runs without steroids, but they were more frequent and went farther because of steroids. Likewise, a wet road doesn’t necessarily cause car accidents (going too fast, abrupt turns, etc. contribute), but it sets up the conditions that make accidents more likely and severe.

It’s been warmer in the past…
This statement is absolutely true, but it’s often used to imply that humans aren’t changing climate now or that warming is no big deal, both of which are ill-conceived. Nature can and does change climate, but this doesn’t prove that humans can’t as well (the evidence listed above shows we have in fact). As an analogy, there have been forest fires for millions of years, but that doesn’t mean humans can’t cause those too. Also, while the world has been warmer in the past, it was a different world when it was, with higher sea level and different rainfall and vegetation patterns. The question isn’t if the earth can cope with climate change – it has for four billion years and will again – the question is how well we’d do with unchecked global warming, which is less certain since we haven’t experienced a big climate change before. One last, more subtle point that is often missed by people downplaying current warming by highlighting big past climate changes is that the larger climate variability has been in the past in response to natural climate forcings, the larger it likely will be in response to anthropogenic forcings.  

Who’s to say what the right climate is?
This may be a fun philosophical tangent, but it’s not too relevant to real-world concerns on the ground. What makes today’s climate the ‘right ‘ one is that it’s what civilization is adapted too. Right or wrong, several feet of sea level rise would flood lots of coastal cities and populations around the world and be a pretty big challenge for society.

Water vapor is the most important greenhouse gas, so CO2 doesn’t really matter
Water vapor (and clouds) does account for the majority of earth’s greenhouse effect. The key thing though is that water vapor is a condensing greenhouse gas and so depends on the climate, whereas CO2 is a noncondensing greenhouse gas and therefore independent of climate. What this means is that if CO2 were slowly pulled out of the atmosphere, it would get a bit colder, which would cause some water vapor to condense and rain out (because cold air holds less water vapor than warm air) weakening the greenhouse effect further and making it colder still. If we continued pulling CO2 out, most of the vapor vapor would eventually condense out of the atmosphere, collapsing the earth’s greenhouse effect and plunging the planet into a snowball state. The way to think about it is that CO2 (and the other noncondensing greenhouse gases) provides the stable skeleton on which the global greenhouse is built, and so in this sense is the most important greenhouse gas out there.

Sea level might catastrophically jump and flood our cities without warning
While we can’t fully rule out the possibility of a catastrophic collapse of the West Antarctic Ice Sheet (which contains 10 feet worth of sea level), sea level rise will probably be a more gradual, chronic condition than doomsday scenarios might suggest, creeping higher and higher up our coasts each decade. Most scientists regard 6 feet of sea level rise as the absolute maximum conceivable amount this century, and think a few feet is probably most likely. The basic reason for this is simple – the Greenland and Antarctic Ice Sheets are absolutely gigantic and so need quite a long time to catch up with the warming climate. Now the less comforting flipside of this statement is that sea level will likely continue rising for centuries after we stop emitting carbon and stabilize the climate. The current best estimate for the size of this sea level commitment is about 7.5 feet of long-term sea-level rise for every 1°C we warm the planet, which if you do the carbon math, comes out to committing ourselves to roughly an additional foot of sea-level rise for every decade of continued carbon emissions at the current rate. You can actually map out what cities are destined for eventual drowning depending on our carbon emissions over the next stretch of decades, which is a bit alarming, though I don’t think irrationally alarmist. Perhaps more important to us in the near-term will be the effects of storm surges riding on top of 1 or 2 or 3 foot higher seas (e.g., think Hurricane Sandy), which will put areas that are currently above high-water marks into flood-risk zones. A final, less often considered point is that sea level rise won’t be uniform around the globe. The ocean isn’t just a bathtub that fills up with water the same everywhere; changes in ocean currents, water temperatures and salinity, and the weakening pull of gravity on the sea surface from shrinking ice sheets means sea level will rise faster in some places and slower in others. For example, sea level rose several times faster along the U.S. east coast than the global average over the past 50 years.

A cold year or a cold spot on the map disproves global warming
Global warming refers to a long-term rise in the average temperature of the planet over decades; this is the time scale at which the effects of rising greenhouse gases begin to clearly outweigh natural variability within the climate. Year-to-year fluctuations are weather, not climate, and so can’t be taken to say much of anything about climate change by themselves. E.g., A dip in the Dow Jones one week or even the current recession doesn’t mean that the global economic growth of the past century has ended, and a cold spell in May doesn’t mean that summer isn’t coming. And the reason that one place getting colder or one glacier getting bigger doesn’t disprove global warming is that they aren’t global. Global warming just means that there’s more heat in the climate system overall, not necessarily that every single place is warmer; if climate change shifts the wind patterns so as to blow more cold air over a particular spot, it might well get colder despite warming occurring at most other places around the world.  So, while it’s tempting to look out the window on any given day to assess the state of global warming, the reality is that one place and one time have little connection to climate change. A good or bad day of business for the hardware shop down the street obviously doesn’t tell you a whole lot about the state of the global economy either.

Just a note of caution: keep an eye out for the exact phrasing people use when discussing climate change, as subtle tweaks in language can really change the meaning and veracity of a statement. For example, ‘catastrophic’ is often slipped in front of ‘global warming’, and this makes a big difference. Someone denying catastrophic global warming isn’t denying global warming (which is sensible since it’s pretty hard to reasonably deny), they’re just casting doubt that it will amount to a ‘catastrophe.’ This is a difficult statement to really prove or refute (and so a clever rhetorical shift, whether someone is a climate alarmist or denier) because of uncertainties in e.g., climate tipping points, our adaptability, etc. and also because it’s a bit subjective and so gets us into semantics as much as science – one person might view coastal inundation, declining crop yields, and biodiversity loss as a catastrophe, while another person would see only full-on social and economic collapse as truly catastrophic. Again, think about whether a US debt default would be ‘catastrophic’ – there’s some room in the economic understanding and in the language to spin it a bit in whichever direction you prefer, as people have. Another, probably less important, word switch is between ‘global warming’ and ‘climate change’. For instance, geoengineering by blocking out sunlight could stop global warming, meaning the global average temperature would stabilize, but it probably wouldn’t stop climate change as rainfall patterns might still shift around (which we might care about as much as the temperature part).