The American Heat Wave and Global Warming

Jul 07 2012 Published by under climate

Global warming is a big issue. If we're honest and we look carefully at the data, it's beyond question that the atmosphere of our planet is warming. It's also beyond any honest question that the preponderance of the evidence is that human behavior is the primary cause. It's not impossible that we're wrong - but when we look at the real evidence, it's overwhelming.

Of course, this doesn't stop people from being idiots.

But what I'm going to focus on here isn't exactly the usual idiots. See, here in the US, we're in the middle of a dramatic heat wave. All over the country, we've been breaking heat daily temperature records. As I write this, it's 98 degrees outside here in NY, and we're expecting another couple of degrees. Out in the west, there are gigantic wildfires, cause by poor snowfall last winter, poor rainfall this spring, and record heat to dry everything out. So: is this global warming?

We're seeing lots and lots of people saying yes. Or worse, saying that it is, because of the heat wave, while pretending that they're not really saying that it is. For one, among all-too-many examples, you can look at Bad Astronomy here. Not to rag too much on Phil though, because hes just one among about two dozen different example of this that I've seen in the last 3 days.

Weather 10 or twenty degree above normal isn't global warming. A heat wave, even a massive epic heat wave, isn't proof that global warming is real, any more than an epic cold wave or blizzard is evidence that global warming is fake.

I'm sure you've heard many people say weather is not climate. But for human beings, it's really hard to understand just what that really means. Climate is a world-wide long-term average; weather is instantaneous and local. This isn't just a nitpick: it's a huge distinction. When we talk about global warming, what we're talking about is the year-round average temperature changing by one or two degrees. A ten degree variation in local weather doesn't tell us anything about the worldwide trend.

Global warming is about climate. And part of what that means is that in some places, global warming will probably make the weather colder. Cold weather isn't evidence against global warming. Most people realize that - which is why we all laugh when gasbags like Rush Limbaugh talk about how a snowstorm "proves" that global warming is a fraud. But at the same time, we look at weather like what we have in the US, and conclude that "Yes, global warming is real". But we're making the same mistake.

Global warming is about a surprisingly small change. Over the last hundred years, global warming is a change of about 1 degree celsius in the global average temperature. That's about 1 1/2 degrees fahrenheit, for us Americans. It seems miniscule, and it's a tiny fraction of the temperature difference that we're seeing this summer in the US.

But that tiny difference in climate can cause huge differences in weather. As I mentioned before, it can make local weather either warmer or colder - not just by directly warming the air, but by altering wind and water currents in ways that create dramatic changes.

For example, global warming could, likely, make Europe significantly colder. How? The weather in western Europe is greatly affected by an ocean water current called the atlantic conveyor. The conveyor is a cyclic ocean current, where (driven in part by the jet stream), warm water flows north from the equator in a surface current, cooling as it goes, until it finally sinks and starts to cycle back south in a deep underwater current. This acts as a heat pump, moving energy from the equator north and east to western Europe. This is why Western Europe is significantly warmer than places at the same latitude in Eastern North America.

Global warming could alter the flow of the atlantic conveyor. (We don't know if it will - but it's one possibility, which makes a good example of something counter-intuitive.) If the conveyor is slowed, so that it transfers less energy, Europe will get colder. How could the conveyor be slowed? By ice-melt. The conveyor works as a cycle because of the differences in density between warm and cold water: cold water is denser than warm water, so the cold water sinks as it cools. It warms in the tropics, gets pushed north by the jet stream, cools along the way and gradually sinks.

But global warming is melting a lot of artic and glacier ice, which produces freshwater. Freshwater is less dense than saltwater. So the freshwater, when it dilutes the cold water at the northern end of the conveyor, it reduces its density relative to the pure salt-water - and that reduces the tendency of the cold water to sink, which could slow the conveyor.

There are numerous similar phenomena that involve changes in ocean currents and wind due to relatively small temperature variations. El Nino and La Nina, conveyor changes, changes in the moisture-carrying capacity of wind currents to carry - they're all caused by relatively small changes - changes well with the couple of degrees of variatio that we see occuring.

But we need to be honest and careful. This summer may be incredibly hot, and we had an unsually warm winter before it - but we really shouldn't try to use that as evidence of global warming. Because if you do, when some colder-than-normal weather occurs somewhere, the cranks and liars that want to convince people that global warming is an elaborate fraud will use that the muddle things - and when they do, it'll be our fault when people fall for it, because we'll be the ones who primed them for that argument. As nice, as convenient, as convincing as it might seem to draw a correlation between a specific instance of extreme weather and global warming, we really need to stop doing it.

42 responses so far

  • Shecky R says:

    glad you wrote this... it needed to be said. Anthropogenic climate change seems clear from any number of sources of evidence... but immediate weather isn't one of them.

  • Zuska says:

    Thanks for this post Mark. I can share it with some friends who've asked questions along these lines.

  • Markk says:

    Many respected climate scientists (e.g. not denialists etc) say that the global conveyer explanation of European Temps is a myth. Here is an example from the Earth Observatory of Columbia University (must be near you?)

    http://www.ldeo.columbia.edu/res/div/ocp/gs/

    This idea was advanced by an American military guy many years ago and has become mythically incorporated, but current models really show there is only small effect from the conveyer, it is just regular ocean vs land under the air mass that is generally responible for the difference in these models.

  • Agreed that any particular instance of extreme warm weather is not evidence of global climate warming. But my understanding is that recent increases in the frequency of extreme weather *is* probably a result of and evidence of global climate warming.

    • MarkCC says:

      Yes - increases in the frequency of extreme events is something that we would predict as a result of warming. Warming increases the amount of energy in the atmosphere, which can lead to all sorts of extreme events. Even a tiny increase in the global average temperature represents a huge increase in the amount of energy in the atmosphere!

      But we still can't really conclude that any specific event is the result of warming. We can say that the increase in extreme events is probably a result of warming - but that's a very different thing than saying "global warming caused the Colorado wildfires".

      • If you had some advanced technology that could instantly eliminate all anthropogenic increases in atmospheric GHGs, then temps would drop a small amount within a week or so on land:

        http://rabett.blogspot.com/2012/07/space-aliens-answer-to-greenhouse.html

        This small effect isn't the truly important one, but it is real. So yes, you can say that our GHGs make heat waves worse, both specifically and as a matter of probability.

        Furthermore, specific weather events are part of the evidence of climatic trends. Just as it goes too far, unnecessarily, to say a specific heat wave is proof of climate change, it also goes too far the other direction to say a specific heat wave provides no additional evidence in support of global warming.

        A record cold wave would provide evidence contradicting global warming, but not proof. Occasional record cold outmatched by much more warmth means climate change is happening.

        • LiveWire says:

          A record cold snap would not provide evidence against global warming, unless it was consistent across the planet.
          What we see are record temperatures in BOTH directions, which is evidence of higher energy levels on the planet, leading to lowered entropy in large-scale weather systems, leading to more chaotic weather, which is evidence FOR global warming.

  • Len says:

    There is some evidence that the atlantic conveyor is a minor contributor to Europe's anomalous warm temperatures. Research by Richard Seager suggests the jet stream plays a greater role:
    http://www.ldeo.columbia.edu/res/div/ocp/pub/seager/Seager_AmSci_2006.pdf

    Here's another take:
    http://www.isciencemag.co.uk/features/dont-blame-it-on-the-gulf-stream/

  • This is an excellent article reminding us not to use a single weather event or even a long term trend in one region as evidence for or against global warming.
    The link in Markk's comment was also interesting, but does not effect the main point of the article.
    Steve Challis

  • DavidML says:

    Excellent post. I'm getting so tired of people picking out the outliers and claiming that global warming is real or that it's a hoax based on that. Even worse than the people using outliers are those that make a big deal of basically normal seasonal variation.

    I can understand why the global warming deniers do it (when it's cold). They really don't have much else to go with. But I really can't understand why everyone else does it. Let's say you actually manage to convince someone that's it's real because this summer is extremely hot (at least in the US), then what happens when the next one isn't (and it very very likely won't be)? Or when we have an extremely mild summer? Or an extremely cold winter? It's just a waste to convince someone of something for the wrong reasons. It won't stick. Plus, you know, it's dishonest.

    Seems to me it's far better to focus on explaining how we actually know global warming is happening, even if it does take longer and require more knowledge.

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  • Windy says:

    Wise words from this author as it always ends up to be a he said she said affair in the blog-o-sphere with climate scientists being as evasive as lawyers in answering questions about the correlation between extreme weather events and global warming. It only serves to further divided the public along political lines.

    What is interesting is that even within the camps of those seeking reductions of CO2, there is a divergence occurring as the more academic/intellectual side seems open to natural gas and nuclear energy options while the more radical enviro groups are not. Lost in the bickering is the 1st quarter 2012 EIA report on US CO2 emissions indicating that the US is on track to reduce its 2006 CO2 emissions by 14% by the end of 2012. Keep in mind that Waxman-Markey cap and trade target was a 17% reduction of 2006 CO2 by 2020, and on our current reduction path we will hit 17% by the end of 2013 seven years ahead of the cap and trade target. If W/M cap and trade had been passed the pols would be taking bows for such success.

    There is a rift developing between radical hard line enviros who will never accept CO2 reduction measures without a carbon tax or some sort of economic justice scheme tied to CO2 reduction, and moderate enviros who are focused on GHG reduction and who see hard line policy alienating the public and failing. I'm rooting for the moderates.

    Personally I see technology as the answer because technology has allowed me to reduce my CO2 emissions to 5.6 tons/year without much of a sacrifice in my lifestyle. To me a more personal approach is needed whereby people learn the benefits of reducing energy and fuel consumption via technology. I freed up $1,500.00/year in energy and fuel costs again with little sacrifice. All people can relate to reducing energy and fuel expenses regardless of politics. As new technology comes out, like $1.50/watt installed PV solar, people will willingly convert because they will save money doing so. If we can move the discussion away from politics and the need to blame someone for the problem, we would be much more able to move forward towards a solution.

  • ecologist says:

    I have to disagree slightly with Mark. Yes, it is absolutely true that a hot day, or a cold day, is not proof of climate change or its absence. But it is equally wrong to claim that regional (I count the whole of the continental US as regional, not local) events have NO connection to warming. And while it is true that climate change is about long-term trends, not short-term fluctuations, the transition between short-term and long-term deserves attention. That's why people, on the one hand, say things like "X out of the last Y years have been the hottest on record" where, the larger Y gets, the more long-term-ish things get.

    What people have a hard time grasping is that, while global average temperature changes on the order of a degree or two are bad, one of the reasons they are bad is that they include a higher frequency of summers like the one we're seeing now. No, it's not going to be like this every year (or at least it's not predicted to), but get used to it, because it's going to become more common.

    Many people have a hard time grasping probability statements. They won't be helped by dismissing events like record-setting heat on a continental scale as unrelated to climate change because it's not global or long-term enough, any more than they will be helped by claiming that it's "evidence for" global warming. We have a difficult communication challenge.

  • Jim Thomerson says:

    I took climatology and meterology courses in the mid 1950s. At that time climate, either local or world wide, was defined as the average weather for the last 40 years. I see it defined as the last 30 years these days. It is sort of ironic that we get climate change just from the definition, adding last year and dropping the year 41 or 31 years ago.

    • MarkCC says:

      Climate is, loosely, the long-term average of weather over the entire planet. You can look at different aspects of climate using different time periods: the average over ten-year periods, 50 year periods, centuries, millennia. There's no one correct time period - the appropriate time period depends on what kind of analysis you're doing. If you're trying to understand the historical cycle of ice ages in the climate, then looking at a 40 year window is much too short to get a real understanding - the changes you're trying to see are much too slow to show in that window - you need to look at time periods that span at least a couple of centuries (probably much larger than that). On the other hand, if you're trying to observe a relatively rapid change - like, say, the little ice age in Europe a couple of centuries ago, looking at a window of 200 years isn't going to work - the changes are too fast to analyze in that framework.

      For understanding global warming, we look at multiple window sizes - but we necessarily bias towards the shorter term, because we're looking at something fast.

      • Dev says:

        Is all the information gathered via technology/automated? given the importance of the issue and the social/economic changes that the field is looking for it would be a good idea to have independent verification of the data (multiple teams, multiple methods, different tools, and direct human participation).

      • Jonathan says:

        That's not quite right, Mark. It's important to understand that global warming is talking about an average over the planet, and doesn't mean everywhere will be warmer, but the word "climate" doesn't imply a global setting.

        This year's weather where you are is only part of the climate in that region, which involves statistical description (averages are just the start) of the long-term weather. The climate in that region is only part of the global climate.

        (Not really sure that the speed of the changes is the problem with using a short averaging window for looking at ice ages, either.)

  • Walt G says:

    So if one suggests the current heat wave is evidence of a larger warming trend he is not "careful;" if one suggests a hard winter is shows the contrary he is a crank and a liar.

    • MarkCC says:

      Yes.

      You see, there's a very important difference, that goes beyond the simple statement "Weather event X (proves/disproves) global warming". The statement doesn't exist in isolation.

      We live in a world where we've got mounds of data about both weather and climate. And any honest attempt to look at that data in detail produces an undeniable result: our planet is getting warmer. It's long past the point where anyone can honestly look at the data, and conclude that there isn't a warming trend. It's still just barely possible to look at the data, and question whether or not human activity is responsible for that change - but you can't honestly conclude from the data that there is no change.

      If you try to use an anomalous weather event to support something that you know isn't true, you're a liar. If you look at the volume of data, and refuse to accept what it clearly says, you're a crank. If you've never bothered to actually look at the data, and you're creating conclusions from thin air to support your idealogical preferences, then you're a crank *and* an idiot.

    • WxChief says:

      What makes extreme events, such as heat waves, floods, droughts, etc. relevant to climate change is how their frequency of occurrence has changed over time. The area of global coverage of such extremes at any one time over the past 30 years has increased from less than 1 % to around 10 %. Looking at it this way may make it easier to understand the why the chance that you will experience an extreme event has increased because of climate change.

      • LiveWire says:

        The fact that we just had a derecho is unusual, but not particularly important, given the last one occurred over a hundred years ago. However, if we have three more in the next decade, that indicates a larger problem.

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  • VictorE says:

    "A heat wave, even a massive epic heat wave, isn't proof that global warming is real". But given that warming causes more extremes, doesn't an increase in extremes increase the probability that warming is indeed happening? Your use of "proof" suggests a zero/one conclusion, whereas everything here is probabilistic.

    • MarkCC says:

      The point is that to understand warming, we need to look at long-term global trends, rather than at individual events. A heat-wave in the continental US isn't evidence of global warming. But an sustained increase in extreme weather across the globe is.

      It's not just the NA heat wave - in isolation, that means nothing, because extreme weather always happens. But when you start looking in combination at the NA heat wave, droughts, ice-melts - and also at other changes like unusually cold winters in some places - the totality is evidence of global warming.

      That's the point I'm trying to make. Our natural instinct is to focus on our own immediate experiences, and therefore to say "gosh, it's sure awfully hot here, global warming must really be happening". But then, when we get a mild summer - and we will get some mild summers, purely by random chance! - our instinct will be to say "Oh, gosh, this summer hasn't been hot at all, that global warming stuff must be nonsense". Warming is about the total amount of energy trapped in our atmosphere, not about a local weather anomaly. We need to recognize that.

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  • Tim Martin says:

    I liked this post, Mark, and I'm trying to understand the principles behind it. It seems to me that you're saying we have to look at long-term data, not just say "oh it's hot now, ergo global warming," right?

    So what do you think of the conclusions cited here? The National Climatic Data Center has looked at the long-term data in order to say something about the probable cause of the weather we're having now. Would you say this is valid?

    If I were to answer myself, I would say that the weather we're having now is a single data point. Regardless of whether the weather in the US is hotter or colder than normal, there is, in many years leading up to now, a trend of increasing temperature. If this year's weather's happened to be colder than usual, it still would not erase the trend. Most likely, average temperatures would continue to rise. And, if this year's weather is hotter than usual, this, too, could be the result of random chance.

    The NCDC, however, has looked at the weather over a longer period of time (not just this summer), and so the trend they are showing is valid. When they say the odds of the record U.S. heat being a random event are small, they aren't just talking about this summer, but about the past 12 or 13 months.

    How's that sound?

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  • wim says:

    This paper argues that you cán link extreme weather events to global warming (because the more extreme they are, the more unlikely they would happen in a non-warming world):
    http://www.columbia.edu/~jeh1/mailings/2012/20120105_PerceptionsAndDice.pdf

  • _Arthur says:

    Hansen et al. 2012

    A recent paper in PNAS by Hansen et al. (there’s also a recently released discussion paper on the topic) has caused quite a stir. The essential result is that extreme heat (beyond the 2-sigma and even 3-sigma level) has become so much more commonplace, that the only plausible explanation is global warming.

    http://tamino.wordpress.com/2012/08/13/hansen-et-al-2012/

    http://www.pnas.org/content/early/2012/07/30/1205276109.abstract

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  • cyniqu says:

    We put a lot of faith into the data. However, the data itself might be semi corrupt. When the same site is used to register temperature, any area around the thermometer has gone from being semi rural to surrounded by concrete, there will be a natural temperature increase. What this proves that there is such a thing as "the urban heat island effect." I'm not certain that it proves general trends, planetwide.

    Yesterday, our high temperature was 89. Our low-temperature was 70. That should average out to 79.5. But that temperature gets rounded up to 80. 80 is what goes into the record keeping books. Our average high temperature is 86, and our low temperature on average is 65, for an average of 75.5 for the day. Nevertheless, the National Weather Service has reported that the average for today is 75. But because some people wish to maybe cook the books, it was recorded that today was 5° above average. If you do this enough, but enough days, and in enough weather reporting stations, it may be conceivable that the "average temperature" is rising in ways that may seem historically significant. Nevertheless,a 100 year period has never been enough to prove decisive climactic changes one way or the other.

    Arctic core samples, Antarctica or Greenland... And periods of true warming, doesn't it seem that the ice would actually melt, and the evidence for warming would actually disappear.? I know people look at the Arctic ice record the way that others look at tree rings : And when you're dealing with trends over 10 to 20 to 30,000 years ago, the data are not as potentially conclusive has a Tree ring from 35 years ago could be. When one "ice strand" represents tens of thousands of years, it's very difficult to prove or to conclude anything substantial.

    • MarkCC says:

      Yes, people who study climate and are experts in statistics *never thought of any of that*. None of that was ever taken into account. Your amazing genius has overwhelmed me.

  • Tom Moran says:

    MarkCC's response above is not typical of those who embrace the scientific method.

  • Tom Moran says:

    There is no link...none....between *extreme weather events* and warming. Extreme weather events are the creation of the Weather Channel. Super storms used to be called weather.

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