Rodney Viereck, NOAA Space Environment Center
Nearly every day, new evidence is presented showing that the globally averaged temperature of Earth has increased over the last few centuries. According to the Intergovernmental Panel on Climate Change (IPCC), the globally averaged surface temperature has increased by 0.6°C over the last 100 years. There is evidence that not only is the atmosphere warming but the ocean temperatures are increasing as well. The ice cap on the North Pole has become significantly thinner.
The global warming has increased dramatically in the last 20 years. The IPCC report estimates that the 1990s were the warmest years since the beginning of instrumental records in 1861 and that 1998 may have been the warmest year on record. This increase in temperature over the last century is likely to have been the largest 100-year increase in the last 1000 years. Because of these dramatic climate changes of the last 100 years many scientists believe that human activities, such as burning fossil fuels, have contributed to global warming.
Two of the questions that now face scientists studying climate change are…
In order to answer the first question, scientists must answer the second question.
The total energy output of the sun is nearly constant. At the top of Earth’s atmosphere the total irradiance from the sun is about 1366 W/m². Imagine thirteen 100 Watt light bulbs shined all of their energy onto a square meter. During the course of an 11-year solar cycle, the average output of the sun changes by about 1-2 W/m² or about 0.1%. Thus, the solar constant varies between 1365 and 1367 W/m² and is therefore, not really a constant.
In other wavelengths such as the ultraviolet and extreme ultraviolet parts of the solar spectrum, the solar variability can be quite large. In the x-ray wavelengths, the sun can change brightness by a factor of 100 or even 1000 in just a few minutes but these wavelengths only affect the upper reaches of our atmosphere. Figure 2 shows a 5-year sequence of x-ray images of the sun from solar maximum to solar minimum.
It is thought that the total solar output of the sun has changed by larger amounts over longer time scales. There is evidence that the total solar output may have been as low as 1360 W/m² during the 19th century and even lower than that during the 17th century. Thus over centennial time scales, the solar output may have changed by 0.5%.
SOLAR VARIABILITY AND CLIMATE CHANGE
Variability in the amount of energy from the sun has caused climate changes in the past. It is now accepted that the global cooling during Ice Ages is the result of changes in the distribution and amount of sunlight that reaches Earth. During the last Ice Age, the globally averaged temperature of Earth was about 6°C colder than it is today. While this may not sound like much, the effect was to cover large parts of Canada, Alaska, and Siberia with huge sheets of ice up to a mile thick.
Even the climate changes of the 20th century may have a significant solar component. Figure 3 shows comparisons of globally averaged temperature and solar activity. Many scientists find that these correlations are convincing evidence that the sun has contributed to the global warming of the 20th century. Some say that as much as 1/3 of the global warming may be the result of an increase in solar energy. So, while it is becoming clear that human activity is changing the climate today, solar activity may also be contributing to climate change and probably changed the climate in the past.
In order to accurately predict how future human activities will change the climate, it is critical to understand the variability of the natural system. Therefore, even though solar activity may not be the dominant factor in global warming, it is important enough that understanding how the climate responds to small changes in solar irradiance will help scientists predict the climate changes caused by human activity.
The NOAA Space Environment Center (SEC) combines scientific research and an operational Space Weather Center to maintain a vigilant watch on solar activity. SEC’s primary mission is studying the affects of a variable sun on the upper atmosphere and the near-Earth space environment. Monitoring and understanding the solar effects on the middle and lower atmosphere is a new component of SEC’s mission. Present NOAA/SEC activities include monitoring the sun in x-ray and ultraviolet wavelengths as well as sunspots. NOAA recognizes the need for new efforts in this area and will include solar extreme ultraviolet measurements on the next generation of GOES spacecraft and total solar irradiance and solar spectral irradiance measurements as part of its upcoming NPOESS spacecraft mission.
DID SUNSPOTS SINK THE TITANIC?
It is well documented that the early part of the 20th century was much colder than it is today. This can be seen in the plots in Figure 3. A consequence of these colder temperatures is that there are changes in sea currents and temperatures and in the strength and direction of the winds at sea. As a result, large icebergs from the Greenland ice sheet would often drift southward into the Atlantic Ocean and into the shipping lanes between Europe and America. It was much more likely that a vessel would encounter icebergs back in the early part of the century than it is now. This is in part a consequence of a cooler climate 80 years ago.
In a scientific paper, written on the subject of the weather on that night in 1912 when the Titanic struck an iceberg and sunk, E. N. Lawrence concludes that there is a link between sunspots and the icebergs found in shipping lanes in the early 1900s. Figure 5 is a plot from Lawrence’s paper showing the correlation between sunspots and icebergs.
While most scientists would agree that sunspots did not really sink the Titanic, there is significant evidence to show that the cold climate of 1912 may have been in part due to the lower level of solar energy reaching Earth relative to today. The cold climate may have provided the conditions needed for large icebergs to drift far south of Greenland and into the shipping lanes of the North Atlantic. These icebergs were a severe hazard to early ships without radar especially at night when they could not see the icebergs. To state the connection more clearly, increases in globally averaged temperature, produced in part by increased solar and human activity, may have reduced the number of icebergs in the North Atlantic thereby preventing other disasters such as the sinking of the Titanic.