We're sort of ready for Sandy - the combination of three storms that is currently threatening our area. A quick check of my email found several people wondering if this is caused by global warming.
Of course the stock answer is that you can't state with any certainty that a particular weather event is "caused" by a shift in the climate. We do know with a very high degree of certainty that global warming is underway and there is a serious piece of it that is caused by the greenhouse gases that come from our burning of fossil fuels. There are many unknowns - exact mechanisms, the location dependence of future weather events, the location of micro climates, the response of the ecosystem and of human institutions and so on. That makes it a rich, if somewhat terrifying, field of study.
Greenhouse gases are a good thing. A simple calculation of the Earth's mean temperature without them gives up a bit below 0°F rather than the current figure of about 59°F. The Earth would be an icy place and life as we know it impossible. But you can have too much of a good thing. Life has a very complex set of interdependencies and is finely tuned to local environment including climate. Over time the planet's temperature has moved around, but these changes have been slow and evolution and a large number of extinctions of species that couldn't hack it have given us our current world. What exists now is a very swift change - the speed turns out to be much more important than the amount ... but that is a lengthy subject and I meant to comment on how much of the unusual weather event bearing down on the Northeastern US may be from our hands.
One of the best analogies is that of a baseball player who uses steroids. I've been using it for some time1... rather than me writing a few paragraphs here is a video featuring Jerry Meehl of the National Center of Atmospheric Research.
So the mean of distribution of extreme weather events if shifted to the right. There is a lot of fundamental science behind this. One of the major drivers is the extra energy from the warmer climate. It turns out it is mostly stored in the ocean and one of the consequences of a warmer body of water is an increased amount of water vapor in the atmosphere. In fact the roughly 1°C increase in temperature we've seen in the past century has increased the amount of water vapor by about seven percent and most of that has come in the past thirty years. This means there is much more water available for precipitation.2 This translates into much stronger storms, although it doesn't necessarily mean more of them. It turns out it has a tendency to make wet areas wetter and dry areas dryer. We're currently in a phase where many of these effects are relatively linear, but we're beginning to see a few non-linear events that will probably make extreme weather even more common and could increase the rate of temperature change.3
Let's get back to the simple explanation of what a shifting distribution means. I fired up Mathematica to make a few plots. The first shows two gaussian distributions with a mean of 0 and standard deviations of 1.5 and 2.2. Probability is plotted on the vertical axis and the horizontal axis is whatever you are measuring. These simply give you the probability that a measurement of something that has a gaussian distribution is at a certain value. Notice that the variation increases as the standard deviation increases. You might think of a thermometer in your back yard and another in your house. The variation of measurements in your house is going to be small - probably within a few degrees over the course of a year. The variation outside your house (unless you live in San Diego:-) will be much larger. Notice that as the variation gets larger the plot lowers.
The second plot has two gaussians with the same standard deviation (1.5), but with different mean values - one is centered at 0 and the other at 1. One effect of global warming is that it shifts the mean temperature. If temperature drives extreme weather events you can expect to find more of them. Also note that a shift of 1°C over the past century took us from 58° to 59°F ... there are still many opportunities for cold weather events and if precipitation, which is now more likely to happen in large amounts given the additional water vapor in the atmosphere, happens when it is below freezing we get larger snow storms (on average).
Moving on to the third graph we have one gaussian with a narrow standard deviation and the other that not only has a larger variation in measurements, but is shifted a unit to the right. Notice that the probability of events at the left are larger than the first case in the shifted broader distribution and is much higher on the right side. It turns out this is what we are currently seeing in our weather system. The climate models suggest greater variation and a shift and we are measuring just that in our changing weather. The forth graph just shows all three distributions - the shift from the blue to the green distribution is a qualitative view of what is taking place with the climate.
Good luck to all of you in the storm's path. We're going to have to get increasingly good at dealing with the rapidly shifting climate we'll be experiencing over the coming years and decades. There is a good chance your kids and grandkids are going to witness much more powerful storms than you'll see, so forget about those "when I was young" stories.
1 I think the baseball analogy is from Jeff Masters and Tony Broccoli of NCAR... One of those great illustrations of something that is highly technical made understandable by anyone. There should be a prize for people who craft these wonderful communications of science.
2 Only a rough statement as the system is very complex. It can change ocean and atmospheric currents and local climates dramatically. This is the focus of much of the modeling work underway around the world.
3 One interesting non-linear event is the disappearance of the Arctic ice cap. When it exists it reflects most of the sunlight that hits it. Dark sea water, on the other hand, absorbs solar energy and a dramatic shift takes place when the ice disappears.
Sweet Potato and Chickpea Salad
° 2 largish sweet potatoes diced into inch pieces
° 1 can chickpeas (about 15 ounces), drained
° 2 garlic cloves
° 2 scallions chopped
° extra virgin olive oil
° 2 tbl tahini
° zest and juice of 1 lemon
° salt and pepper
° 1 to 2 tbl extra virgin olive oil
° handful of coarsely chopped pecans (toasted if you like)
° Preheat oven to 400°F and line a baking sheet with parchment if you want to protect it
° roast the sweet potatoes for about 20 minutes and then turn them over. Roast until they begin to brown on the surface - about 35 to 40 minutes in my oven
° while the sweet potatoes are roasting, sauté the chickpeas in a pan over medium heat with the garlic and some olive oil until they are dry and warmed throughout. About three minutes for me. Remove the garlic cloves and reserve them for the dressing.
° To make the dressing chop the garlic finely and mix with the tahini, olive oil and lemon juice and zest. Season as necessary with salt and pepper and whatever else you want (I like it just as it is). Set aside
° Combine the finished sweet potatoes and chickpeas in a bowl and toss with the scallions.
° pour the dressing over the veggies, toss, test for seasoning and toss with pecans and serve. This is best at room temperature.