(This exercise is based on Graversen, R. G., T. Mauritsen, M. Tjernström, E. Källén, and G. Svensson. 2008. Vertical structure of recent Arctic warming. Nature 541: 53–56.)
(Note: The reference above links directly to the article on the journal’s website. In order to access the full text of the article, you may need to be on your institution’s network [or logged in remotely], so that you can use your institution’s access privileges.)
Global climate change is occurring; Earth’s surface temperature increased by about 0.6°C during the twentieth century. That change, however, was not uniform: the increase in temperature varied considerably in different regions of the globe. In particular, the Arctic region has experienced considerably greater warming than Earth as a whole.
Why has the Arctic warmed faster than the rest of the planet? One possibility is that pollutants of the ice and snow have darkened the surface of the Arctic, thus decreasing its albedo. The change in albedo could lead to the surface reflecting less heat, thus accelerating warming in the Arctic.
Rune Graversen and other researchers at Stockholm University investigated this hypothesis by examining temperature changes during the winter months (November–February), a time when little or no solar radiation occurs in the Arctic. The lack of solar radiation during this time of the year rules out changes in albedo contributing to long-term patterns of temperature change in this region. The researchers focused on the last 45 years of temperature data taken from land-station observations in the Northern Hemisphere as a whole and the region north of 65°N.
Figure 1 Dark-month (November–February) anomalies of mean temperature relative to the 1850–1900 average as a function of year. Data were obtained from land-station observations. In the main figure, the symbols represent means from individual years, whereas the lines show the temporal evolution when variability over timescales smaller than 20 years has been removed using a wavelet filter. Solid line and open circles are based on observations north of 65°N, while the dashed line and dots are for the entire Northern Hemisphere. Inset shows the smoothed temperature time series for the full instrumental period. The data were provided by the Climate Research Unit (CRU) as a 5° × 5° gridded data set. [Note that the temperature change (anomaly) is determined by subtracting the mean temperature from the reference (1850–1900) from the actual temperature values.]
Question 1
By how much did the average November–February temperature change in the Northern Hemisphere between 1965 and 2005?
Question 2
By how much did the average November–February temperature change in the Arctic region between 1965 and 2005?
Question 3
Based on these facts, what can you conclude about the role that albedo changes play in the enhanced warming in the Arctic?
Figure 2 Average temperature trends around latitude circles for 1979–2001 plotted versus latitude and height for the four seasons. Trends are shown for winter (a, December–February), spring (b, March–May), summer, (c, June–August) and autumn (d, September–November). The linear trends are estimated from monthly mean data using a least-squares fit.
Feedbacks of snow and ice reflecting solar radiation are expected to primarily affect temperatures in the atmosphere closest to the surface, and not have much effect at higher atmospheric levels. The researchers looked at temperature changes at various latitudes and at different levels in the atmosphere. Note that atmospheric pressure declines with elevation from Earth’s surface, though not according to a linear function. At any given altitude, pressure does not change much. It is approximately 1,000 hPa at sea level. Pressure of 700 hPa corresponds to an altitude of about 3,400 meters, pressure of 500 hPa corresponds to about 5,800 meters, pressure of 300 hPa corresponds to about 9,500 meters, and pressure of 100 hPa corresponds about 17,000 meters.
Question 4
At the North Pole during the winter months, where in the atmosphere has the greatest amount of warming occurred?
Question 5
At the North Pole during the summer months, where in the atmosphere has the greatest warming occurred?
Question 6
What do the data from the atmospheric studies imply for the hypothesis that albedo changes are the main factor for the accelerated warming of the Arctic?
