Last week brought some sobering news from the Mauna Loa Observatory where the Keeling family has been dutifully recording atmospheric concentrations of carbon dioxide since 1958. On May 9, measured concentrations surpassed 400 parts per million (ppm). 
Of course, from a scientific perspective, there is nothing particularly significant about the number 400. But cognitively, round numbers have an ability to attract attention as symbolic –and in this case, unnerving- milestones.
For those of us who aren’t accustomed to thinking in terms of parts per million, it is useful to put this number 400 into perspective. Before the Industrial Revolution, atmospheric carbon dioxide concentrations were approximately 280 ppm. When Dr. Charles David Keeling first set up shop on Mauna Loa to collect the daily measurements that comprise the so-called “Keeling curve” (see below), C02 levels were measuring at 315 ppm. A growing number of scientists and climate experts have identified 350 ppm as the safe upper limit (spurring the creation of 350.org).
Perhaps more alarming than the number 400 is the rate at which we got here. Looking at this graph, you can see that atmospheric carbon dioxide concentrations are increasing at an increasing rate. Over the past 10 years, the annual rate of increase in carbon dioxide concentrations has averaged 2.07ppm. This is more than double the rate of increase in the 1960s.
More bad news from the economists…
Climate scientists monitor concentrations of greenhouse gases in the atmosphere in terms of ppm. Economists are more concerned with SCC. The “social cost of carbon” (SCC) measures the present value of the economic damages (in dollar terms) associated with an incremental increase (usually a metric ton) in carbon dioxide emissions in a particular year.
Whereas climate scientists can directly measure concentrations of greenhouse gases in the atmosphere with precise (and really cool) instruments, estimating the SCC is a far more complicated and controversial endeavor. If only we had the equivalent of a non-dispersive infrared instrument to precisely measure the social cost of carbon in the global economy! Instead, economists build integrated assessment models to capture the interactions and feedbacks between the climate system and the economic system. These models (e.g. RICE, FUND, PAGE) are then used to simulate how incremental changes in greenhouse gas emissions affect economic outcomes.
The limitations of these integrated assessment models are well documented. Work to improve and refine these models is ongoing. Importantly, as these models get fine tuned, estimates of the social cost of carbon are being adjusted up.
Take, for example, the influential RICE/DICE models that have figured prominently in SCC calculations. The graph below, taken from a recent paper by William Nordhaus, summarizes how the optimal carbon tax trajectory (a concept closely related to the SCC) has been adjusted as the models are refined.
In 1999, the optimal carbon tax in 2015 was estimated to be in the range of $10 per ton of carbon dioxide. When the model was updated in 2010, the 2015 value increased to $40. The PAGE model provides another important example. When this model was recently updated, the mean SCC estimate increased from $81 to $106.
What is driving these increases? The answer is complicated and involves a host of factors (including how global production is measured, how temperature sensitivities are modeled, assumptions about the effectiveness of adaptation, economic preference parameter choices, etc.). But the punch line is simple: climate change is going to cost more than previously thought. And we should expect to see these upward adjustments continue as researchers account for more of the ways in which climate change damages manifest.
Not all the news is bad
Fortunately, not all trends germane to climate change are moving in the wrong direction. Importantly, whereas estimated costs of emissions are increasing, the costs of some important mitigation measures are coming down. Since 2008, wind turbine prices in the U.S. have fallen by nearly one-third. Reported installed prices of U.S. residential and commercial solar PV have declined 5-7 percent per year between 1998 and 2011. PV system prices fell by more than 26 percent in 2012.
This is not to suggest that steering ourselves onto a more sustainable course will come easy or cheap. It won’t. But as we burn past 400 ppm with no signs of slowing down, the economic case for taking substantive action has never been stronger.
 There is still some uncertainty about whether the 400 ppm threshold was actually exceeded. Whereas NOAA revised its reading to 399.89, a second monitoring program run by Scripps Institution of Oceanography read 400.08.