Ask and ye shall receive. As I got in the shower this morning, NPR ran an interview with NOAA meteorologist Martin Hoerling  . The conversation was about the implications of “Snowpocalypse” vs. the current heat wave. He spoke exactly to the issue of mistaking weather for climate, and how climate models predict that what we think of as rare extremes now will become more commonplace in the century ahead.
Snowpocalypse. We all remember the fun that popular media outlets like Fox News had with that one. The U.S. experienced a few weeks of anomalously cold temperatures and suddenly pundits and flapping heads were declaring the end of global warming. Yeah, that was fun. Good times.
Where are the flapping heads now? This summer has seen anomalously HIGH temperatures in the U.S. The heat wave currently striking much of the nation  has generated health headlines, but few itchy-trigger-finger cries about how the pundits were all wrong in the Winter and global warming is really real. It’s an excellent example of cherry picking data; that practice is a clear sign of a poor understanding of science. Snowpocalypse fit a comfortable narrative, in which climate change is a hoax and so can be easily dismissed. Our present triple digit temperatures don’t fit that narrative; those same people seem to have lost their high-paid voices.
The failure of popular media to provide good reporting on science is a failure of our community to train and educate, so that one is on us. That said, it’s never too late to do a better job. It’s bad science to cherry pick hot temperatures as evidence of global warming, just as it’s bad science to cherry pick cold ones as evidence against it. In reality, the evidence that global climate change is happening is a 150-year march of average global surface and atmospheric temperatures up, up, and up. Local anomalies are inevitable, and should not be read into too deeply. But we can use this moment of scorching temperatures to ask how widespread the heatwave actually is.
Right now, in the Northern Hemisphere, it is summer, while in the Southern Hemisphere it is Winter. This is because of the tilt of the earth, at this part of our orbit, the Northern Hemisphere is tilted toward the sun and the Southern Hemisphere is tilted away; in 6 months, the opposite will be true when we are on the other side of the sun.
We can revisit the excellent NASA website that shows you current monthly average temperatures versus averages for the same region from 1951-1980 (or any other period you like) . I generated another map based on the NASA GISS data (see below).
The anomalously high temperatures began in late May for the Southwest and continued through June, spreading to other parts of the U.S. But we also see that June was anomalously hot for all of Europe, Russia, and Asia (anywhere from 0.5-5 degrees Celcius too high). Nearly all of the lower 48 of the U.S., excluding the Pacific Northwest, experienced temperatures that were 1-4 degrees Celcius hotter than the average from 1951-1980. What’s more interesting is the Southern Hemisphere, where most of South America and Africa experienced higher-than-average Winter temperatures. Australia and Antarctica are the only continents seeing anomalously low temperatures, much like in February the U.S. was the only place experiencing anomalously low temperatures. Back then, Europe was seeing anomalously high temperatures leading to early snowmelt and suboptimal skiing conditions.
Snowpocalyse is an excellent example of how the popular media is poorly trained to understand science, especially the difference between short-term and long-term effects. Weather is a short-term effect; it is hard to predict more than a few days out due to the large number of factors that can lead to a local weather pattern (the earth is big; a storm that stretches across several states is still small on the scale of the forces at work across the earth). Climate is a a long-term effect; there is also a large, average set of factors that affect it, but climate scientists have demonstrated more accuracy in seeing the outcomes of changes in climate.
However, one doesn’t need predictions to say the earth is warming; the data are clear. Opinion leaders in the media want easy answers to this serious challenge, like extra snowfall signaling the end of the phenomenon. Alas, earth is not so small that we can take comfort from a few weeks of snow on one continent. Earth is a vast and complex system, one that’s getting quite hot.
About a year ago, I made the following prediction  about European temperatures in the wake of the eruption of Eyjafjallajökull in Iceland :
So I make a prediction, as a very amateur climate science armchair guy. I predict that Europe will experience unusually cold temperatures in the next year. In the next days, this will likely be a combination of the lack of contrails and the ash, but once air travel is restored I’ll wager that local temperatures in the European continent will still go unusually cold this coming year due to the ash. (From )
Well, I was only briefly right . . . maybe. I grabbed some maximum temperature data for a monitoring station in Oxford, UK, which according to the composite plume map  was CLEARLY affected for weeks after the eruption. I obtained date and maximum temperature information from .
The data are shown in the plot below. The hump on the left is the winter-summer-winter cycle for 2009, and the hump just to the right of middle is the winter-summer-winter cycle for 2010. The 2011 data is the trend on the far right. The red area indicates the 30 days after the eruption. Time on the x-axis is given relative to 4/14/2010, the day the ask cloud appeared.
The data shows that about 10 days after the ash cloud began, maximum temperatures at the monitoring station started to decrease for about 10-15 days. By about 45 days after the eruption, temperatures were already climbing back to summer normals. Comparing to 2009 data, there aren’t too many other differences between 2010 and 2009. So while there was a neat little downward blip in the temperatures, things seem to restore fairly quickly.
It would be neat to see a really deep quantitative analysis to know if there were more subtle effects. But I’d say I missed the mark on that call. That’s what I get for extrapolating from the volcanic eruption the Ben Franklin experienced while in France near the end of the American Revolution. Damn you, Ben Franklin.
I hear a lot of interesting things when I play the “fly-on-the-wall scientist.” Most statements uttered casually between friends can be tested scientifically; at the very least, research has already been done and one only needs to dig a little to find out whether the statement is true. There are many things in life that can be demonstrated true and false. Grab a bowl; let’s pop a serving of buttery science!
Carbon dioxide (CO2) levels lag behind changes in global temperature. Therefore, CO2 is not a greenhouse gas and/or other causes are at work in global climate change and therefore the current changes are not human-induced (“anthropogenic”).
After concluding a review and Q&A session for my introductory physics course, I had to remain late at SMU until Jodi returned from a formal dinner event. I was killing time (trying not to think about the norovirus that was playing havoc with my intestines that same day) when a conversation between two colleagues at the other end of the room caught my attention. One of my colleagues was arguing, apparently as he usually does, that anthropogenic climate change has little or no scientific basis. To support his claim, he repeated a statement I have heard before from anthropogenic climate-change deniers (ACCDs): CO2 levels are a lagging, not a leading indicator (that is, the rise in CO2 levels TRAILS the change in temperature), and therefore CO2 is either not a greenhouse gas or global climate change is induced by factors other than CO2, so humans cannot be responsible.
I decided that this was a real opportunity to me to investigate the background of this statement. So here we go!
A look at the chart
Here is a great example of a plot that gets referred to by many ACCDs (reproduced from ).
Above, we see a lot of data being shown at once. Concentrate on a, b, and c. Curves a and b are the levels of CO2 and CH4 (methane), both greenhouse gases (that is, these are gases that can trap heat). We see the levels rising and falling over long periods of time, but if we compare to curve b, the isotopic atmospheric temperature, we see that changes in temperature for warming lead the changes in CO2 and CH4. For a slightly marked up version of this plot which helps see where warming trends fall and how insolation changes match to those, see the appendix below.
Why can warming ever lead greenhouse gases?
Before we proceed, it’s definitely worth understanding how the researchers came to these results. They obtained and studied ice cores from Antarctica. Ice traps bubbles of atmospheric gas, effectively encasing a snap-shot of atmospheric conditions over vast periods of time. Studying the trapped gases taught the researchers about atmospheric CO2 and CH4 levels. How do they determine temperature changes? They compare the level of Deuterium present at each level of the ice core. Heavy water isotopes are always present in water; however, water vapor will tend to contain lower levels of heavy isotopes and high levels of light ones; precipitation will contain more of the heavy isotopes and lower levels of the light ones. Changes in deuterium levels therefore are a proxy for temperature, and changes relative to modern temperatures (and the Deuterium levels at present) can be used to chart temperature back in time.
Finally, what is “insolation” shown in curve e in the above plot? According to Wikipedia :
Insolation is a measure of solar radiation energy received on a given surface area in a given time. It is commonly expressed as average irradiance in watts per square meter (W/m2) or kilowatt-hours per square meter per day (kW·h/(m2·day)) (or hours/day).
As the Earth’s relationship to the sun changes due to the eccentricity of the orbit, the tilt of the rotation axis, and the precession of the earth’s orbit, so does the amount of radiation received per unit area. There is a fairly clear understanding of how this change influences Earth’s climate in regular cycles, called “Milankovitch cycles.”  The above graph shows that insolation, specifically increases in Joules striking the Earth, can initiate warming trends.
Increases in solar radiation can initiate warming trends by causing ice-age glaciers to recede. Melting/receding glaciers reduce Earth albedo – the property of reflectivity that, rather than trapping solar radiation simply reflects it back into space. Reduced albedo means more heating. As the heating starts to increase, CO2 and CH4 are released from storage on or in the earth (in sea water, for instance, where under cooler temperatures the gases remain trapped in the water). We see then how CO2 and CH4 levels can begin to rise after warming has begun. This then creates a feedback loop; higher levels of heat-trapping gases trap more heat, and heating begins to accelerate, causing more glacier to melt. This defines the sharp warming we see in the graph above. Subsequent re-cooling of the earth can take tens of thousands of years, after only a couple of thousand years of this warming. Then the whole thing repeats again. This, according to Antarctic ice samples, has been repeating every 100,000 years or so for the last 400,000 years.
It is important to note that the lead that temperature has on CO2 levels, historically, is a small one. Current estimates range between 200-1000 years of leading. However, the warming trend lasts thousands of years – so it does not end when CO2 and CH4 enter the atmosphere in increased amounts, it continues strongly thereafter.
What’s happening now?
So we see how insolation (the change in earth’s reception of energy from the sun) can lead to warming (decreasing glaciation), which then releases greenhouse gases after the warming is initiated and begins a few thousand year runaway period of warming.
Is that what is happening in the current period? Are ACCDs right?
The answer, quite simply, is no. Here is why.
First, look at the above graph. Our current era is at the left of the graph. We see that solar insolation is at a minimum right now – that is, we are in a period where energy from the sun is at a low-point. Previously, warming trends were always initiated on the increasing side of insolation, not on the down-side or at a minimum. So already we see a distinction between the last 400,000 years and the current warming trend.
Second, atmospheric CO2 levels have not lagged the current warming – they have led it. This is very clear in all of the data that has been analyzed looking at CO2 levels and global temperature over the past 200 years, since anthropogenic expulsion of CO2 into the atmosphere began to ramp during the American and European industrial revolutions. The story is told in the CO2 data shown in the insets of the figure below.
IPCC Report 4, Working Group 1, Figure 3.6: Greenhouse Gas Concentrations
The above figure is taken from Ref. . We see that rapid increases in atmospheric CO2 levels (the top plot) began in the 1800s and have continued unabated since. The real up-tick in levels began in 1900, when the slope of the change increased. The next increase in slope was then in about 1955-1960. Let’s then compare that to when temperature changes began over the same period. The figure below, also from , shows these relative to the average temperature between 1850-2006:
Whether we look at global average temperature or just the average in the Northern or Southern Hemispheres, we see the same trend. Temperature seemed to be going in regular cycles around a flat average from the start of the graph (1850) until about 1920. Then in 1920, the average around which regular cycles occurred obtained a new slope – no longer zero (flat), but positive (increasing). From about 1920 to 1980, a period of 60 years, the global average temperature increased by 0.2 degrees Celcius. From 1980-2006, a period of 26 years, global average temperature increased by an additional 0.2 degrees Celcius. The time to double the temperature change cut in half between 1920-1980 and 1980-2006. That’s stunning.
But CO2 levels began increasing long before these temperature changes took off. So in the current era of warming, CO2 is driving the change and not the other way around. CO2 now leads temperature.
Is that CO2 really from human activity?
A final statement that ACCDs might make about all of this is the following: so what . . . there’s no proof that this CO2 comes from human activity, so maybe this is just a natural cycle where CO2 happens to lead temperature for a change.
Actually, there is a clear physics answer to this question: isotope ratios. I have commented on this before  based on studies like those in reference . CO2 that remains in the atmosphere for a very long time, or which is present in biological material on the surface of the earth, is constantly exposed to cosmic ray radiation. This radiation can generate the isotope of stable carbon-12, unstable carbon-14, in a ratio that is actually easily measurable to high precision. In other words, CO2 that derives from close to the surface of the earth has a larger C-14 content ratio than carbon that is more shielded from cosmic ray radiation.
Carbon that has been underground for a long time, in the form of fossil fuels, is just such a kind of carbon that will have a lower C-14 level. By studying the level of C-14 in the air trapped in tree rings and other sources, we can watch to see if over time the atmosphere loses C-14 as more C-12-rich fossil-fuel carbon is pumped into the air. Sure enough, this is exactly what is seen. C-14 is a fingerprint, and we observe that since the 1800s the level of C-14 has decreased dramatically in the atmosphere. This tells us that less radiogenic carbon is entering the atmosphere – just the kind of carbon obtained from deep sequestered fossil fuel. The only species burning lots of deep-sequestered carbon is humans, and thus the modern CO2 levels are anthropogenic in origin.
So, does CO2 lag temperature? Yes, it can, and historically it has. The ice core record is clear on that. However, in the modern warming cycle CO2 and other gases have led the warming trend. In addition, that CO2 is rich in C-12 but depleted in C-14 because it comes from reserves deep in the earth. Anthropogenic warming is the status quo, and acting otherwise is wishing your doctor hadn’t diagnosed you with cancer and shopping around for a doctor that will say otherwise.
4/17/2011: Based on a reader comment, I produced an annotated version of the Vostok Ice Core data plot. The blue lines help you to see how significant warming trends match to locations on the insolation curve. I made the plot “blind”: I zoomed way in on the temperature curve, marked the start of significant warming trends with blue lines, and when I drew all lines I zoomed out and extended the lines to the axes. That way I wouldn’t only choose warming trends that definitely correlated with high radiation from the sun. The blue lines show that this appears to be the case, but any bias I possess should not be a factor in that relationship.
 J. R. Petit, J. Jouzel, D. Raynaud, N. I. Barkov, J.-M. Barnola, I. Basile, M. Bender, J. Chappellaz, M. Davis, G. Delaygue, M. Delmotte, V. M. Kotlyakov, M. Legrand, V, Y Lipenkov, C. Lorius, L. Pepin, C. Ritz, E, Saltzman, and M. Stievenard, “Climate and history of the past 420,000 years from the Vostok ice core, Antarctica,” Nature 399 (1999) 429-436
 http://adsabs.harvard.edu/abs/1981E&PSL..53..349S [“Atmospheric C changes resulting from fossil fuel CO2 release and cosmic ray flux variability.” Stuiver, M. and Quay, P. D. Earth and Planetary Science Letters, vol. 53, no. 3, May 1981, p. 349-362. ]
Atmospheric14C changes resulting from fossil fuel CO2 release and cosmic ray flux variability