Introduction to Climate Science (Schmittner)

Observations

Although Earth’s climate is currently changing rapidly relative to past changes, in most regions where we live changes are slow enough that we do not notice them directly during our daily lives. However, older people may have noticed changes during their lifetimes and in some regions changes are larger and more obvious than elsewhere. In this chapter we will discuss some observations from the past 100 years and data from regions that are particularly sensitive to climate change, where the most dramatic effects have occurred. This will not be a comprehensive documentation of existing observations. Additional observations will be discussed throughout the remainder of this book.

a) Atmosphere

Observations show that climate is changing on a global scale. Surface air temperature data, averaged over the whole Earth, indicate warming of 0.7-0.9°C over the last ~100 years (Figure 1). But that warming was neither steady nor smooth. From 1880 to about 1910 there was cooling, followed by warming until about 1940. After this slight cooling or approximately constant temperatures were observed until the 1970s, followed by a rapid warming until the present. Each year’s temperature is somewhat different from the next. Not all of these year-toyear changes are currently understood, but natural variations, which are not caused by humans, do play a role. For instance, strong El Niño years such as 1997-98 or 2015 show up as particularly warm years, whereas La Niña years such as 1999-2000 or 2011 show up as relatively cool. El Niño and La Niña, also known as ENSO (El Niño/Southern Oscillation), is climate variability in the tropical Pacific that also impacts many other regions of the Earth.

Analysis of thousands of temperature records into an estimate of global mean temperature change such as Fig. is not trivial. Issues such as changes in station locations, instrumentation and data coverage have to be taken into account. The fact that five different groups analyzing the data using different methods come to the same conclusions suggests that the results are robust. The reliability of the data has been demonstrated and it has been shown that station locations (e.g. urban versus rural) don’t matter. See this blog for more discussion. The global warming trend has made the probability of warm and extreme warm temperatures more likely and the probability of cold and extreme cold temperatures less likely. E.g. extremely warm (more than 3°C warmer than the 1951 to 1980 average) summer temperatures over Northern Hemisphere land areas had only a 0.1% chance of happening from 1951 to 1980, whereas during the decade from 2005 to 2015 the chance of those extreme warm temperatures to occur was 14.5%. Conversely, relatively cold summer temperatures (less than 0.5°C cooler than the 1951-1980 average) that happened about every third year from 1951 to 1980 only occurred 5% of the time during 2005 to 2015. See this article and this blog (Figure 5) for additional information and graphs.

In the Antarctic sea ice experiences an even larger seasonal cycle but it has changed less over the last 40 years compared to the Arctic. Southern hemisphere sea ice has slightly increased by about 12 % or 0.5×10 km in late austral summer and 2.4 % or 2.3×10 km in austral winter. Note that year-to-year fluctuations are larger in winter than in summer and that the long term trends in the Arctic are much larger than the short-term fluctuations, whereas in the Antarctic the long-term trends are similar to the short-term fluctuations and thus less statistically significant. A trend is statistically significant if it is larger than the errors.