The modern temperature record
Observations of local temperature and precipitation only date back to the 17th century and were generally confined to NW Europe. More widespread observations began in earnest in the late 19th century, albeit not uniform over the globe. Throughout the 20th century, more observations were taken across the globe including by ocean buoys and ships navigating the ocean waters. In the 1970s, satellite observations began to record measurements of global temperature in a more spatially representative fashion and provide an independent measure of global temperature change. Given the challenges in trying to spatially depict global temperature with the number of moving parts, several government research groups and independent scientists have constructed global temperature time series. Despite their different approaches, all groups converge on the plots shown below that depict an overall increase in global surface temperature of 0.6 to 0.9C over the past 100-150 years.
Time series
This warming has been observed both over land and ocean, and although not spatially homogenous, is generally found across most of the globe. If you take a closer look at the time series you should note several things:
(1) The warming is not monotonic. In other words, each year is not warmer than the previous year. Likewise, each decade is not warmer than the previous decade. However, over the long haul, it is tough to argue that there has not been an increase in temperature.
(2) Much of the warming has been observed since 1975. Prior to this time period, there was nominal change in global temperature.
(3) The past 10-15 years show no obvious change in global mean temperature.
Looking at the graph, what can you conclude about the temperature of 2008 and 2011?
a) They were the coolest years since 2000
b) They were warmer than any year before 1990
c) Global warming has ended
d) Worldwide CO2 emissions dropped significantly in 2008 and 2011
e) Temperatures in 2008 and 2011 are consistent with global warming
Answer: The correct answers are a), b), and e).
But why did the temperature decrease in 2008 and 2011? What factors might be response for temperature changes that occur on such short time scales?
It may be convenient for some to view climate change over short time periods; however, this is very short sighted as we can see by the visualization below.
(1) The warming is not monotonic. In other words, each year is not warmer than the previous year. Likewise, each decade is not warmer than the previous decade. However, over the long haul, it is tough to argue that there has not been an increase in temperature.
(2) Much of the warming has been observed since 1975. Prior to this time period, there was nominal change in global temperature.
(3) The past 10-15 years show no obvious change in global mean temperature.
Looking at the graph, what can you conclude about the temperature of 2008 and 2011?
a) They were the coolest years since 2000
b) They were warmer than any year before 1990
c) Global warming has ended
d) Worldwide CO2 emissions dropped significantly in 2008 and 2011
e) Temperatures in 2008 and 2011 are consistent with global warming
Answer: The correct answers are a), b), and e).
But why did the temperature decrease in 2008 and 2011? What factors might be response for temperature changes that occur on such short time scales?
It may be convenient for some to view climate change over short time periods; however, this is very short sighted as we can see by the visualization below.
Solar Cycles?
The primary energy input to the Earth system is the sun. We have demonstrated that glacial-interglacial cycles are associated with variations in the seasonal and geographic distribution of solar energy to the planet. It turns out that the amount of energy emitted from the sun also changes over time. We have measured the frequency of sun spots for over 400 years and using modern record know that periods of high sun spot activity correspond to increases in the amount of energy emitted from the sun. A view of sunspot numbers over the past 70 years reveals an apparent 11-year cycle in solar activity. Do you observe any similar behavior in the plots of global mean temperature? The additional amount of energy the Earth receives from the sun during a solar maximum (peak sunspot activity) versus a solar minimum (nadir in sunspot activity) is approximately 0.2 Watts per square meter. Directly, this would have a minor influence on global temperature that would not be discernible against the backdrop of natural climate noise. However, scientist do believe that solar activity has had some impact on observed changes in global temperature. We'll revisit this in the next section.
Volcanic Aerosols
Huge volcanic eruptions, typically tropical stratovolcanoes can cool Earth by injecting ash and tiny sulfate-containing particles into the stratosphere called aerosols. The resulting haze shades the sun for a year or two after each major blast and results in a slight increase in global albedo. This increase in global albedo reduces the amount of solar radiation that heats the planet and hence a short-lived minor cooling has been noted following these large eruptions. Relatively few large eruptions were noted in the first half of the 20th century. A couple recent volcanic eruptions, El Chichon (1983, Mexico) and Mt. Pinatubo (1991, Philippines) had a notable influence on climate and reduced global temperature by a few tenths of a degree for 1-2 years.