There are many ongoing signs that the planet is heating up, even “on fire.”
In the western region of North America, the prolonged drought has led to high temperatures and many wildfires, from Canada and the Northwest earlier this summer to California more recently. The Pacific is very active with hurricanes, typhoons and tropical cyclones, and with several damaging hits in Japan, China and Taiwan, in particular. So far, by contrast, the Atlantic tropical storm season is quiet.
Globally, surface temperatures have been setting record high values (see figure below). US temperatures this year are well above normal as a whole, running 1.7 Fahrenheit above the 20th-century average (through July; the 10th highest on record). However, precipitation has been well above average in much of the country outside of the West, making temperatures lower than they otherwise would have been (owing to more cloud and evaporative cooling).
So what’s going on? Increased warming is expected because human activities are leading to increases in heat-trapping greenhouse gases, mainly carbon dioxide from burning of fossil fuels. And indeed, the global mean surface temperature (GMST) has been rising fairly steadily: every decade after the 1960s was warmer than the one before, and the decade of the 2000s was the warmest on record by far; see figure.
At the same time, it is readily apparent that there is variability in GMST from year to year and decade to decade. This is expected and known to arise largely from internal natural variability. While the rate of surface temperature increase has been mostly upward from about 1920 and the recent rate is not out of step overall, there are two hiatus intervals with much lower rates of temperature increase. The first was from about 1943 to 1975, and the second was from 1999 to 2013.
In a paper entitled Has There Been a Global Warming Hiatus?, I find that natural variability through interactions among the oceans, atmosphere, land and ice can easily mask the upward trend of global temperatures. For climate scientists to improve climate models, better understanding of these variations and their effect on global temperatures is essential.
The warmest year in the 20th century was 1998. However, since then there has been an apparent absence of an increase in GMST from 1998 through 2013. This has become known as the “hiatus.” While 2005 and 2010 GMST values slightly exceeded the 1998 value, the trend upwards slowed markedly until 2014, which is now the warmest year on record. Moreover, there are excellent prospects that 2015 will break that record – the past 12 months through June 2015 are indeed the warmest 12 months on record (see figure). It looks like the hiatus is over!
El Niño and Pacific Decadal Oscillation (PDO)
A closer look at the events during these hiatus periods sheds light on the role of natural variability on the long-term trend of global warming.
The year 1998 was the warmest on record in the 20th century because there was warming associated with the biggest El Niño on record – the 1997-98 event. Prior to that event, ocean heat that had built up in the tropical western Pacific spread across the Pacific and into the atmosphere, invigorating storms and warming the surface especially through latent heat release, while the ocean cooled from evaporative cooling.
Now, in 2015, another strong El Niño is under way; it began in 2014 and has developed further, and in no small part is responsible for the recent warmth and the pattern of weather around the world: the enhanced tropical storm activity in the Pacific at the expense of the Atlantic, the wetter conditions across the central United States, and cool snowy conditions in New Zealand.
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