July and August are typically about 3.6ºC or 6.5ºF warmer than December and January. August is typically 1.8°C or 3.24ºF warmer than the average annual temperature. Above image shows how much higher the temperature was for selected months, compared to the annual global mean for the period 1980-2015. Will August 2018 be the hottest month on record?
Numerous temperature records have fallen across the world recently. Heat stress hazard is high under conditions of high surface air temperature and high relative humidity. When looking at heat stress hazards, it's therefore important to look at surface air temperatures over land, i.e. the temperature of the air above the land surface.
Fire hazard is high under conditions of hot and dry soil and strong wind. When looking at fire hazards, it's therefore important to look at land surface temperatures, reflecting how hot the surface of the Earth would feel to touch in a particular location. The map below shows land surface temperatures.
When calculating how much warmer it is now, a number of things must be taken into account:
What baseline is used and how is the temperature at the baseline calculated? In the image at the top, the baseline is 1980-2015, which is a very recent period. When using a preindustrial baseline, anomalies could be more than 0.6°C higher than when using the 1951-1980 baseline that NASA normally uses.
- Surface temperatures or surface air temperatures?
Above map shows land surface temperatures. As said above, this is different from surface air temperatures over land that show the temperature of the air above the land surface.
Similarly, sea surface temperatures indicate the temperature of the water at the surface. Sea surface air temperatures, on the other hand, are slightly higher, they are measurements of the air temperature just above the surface of the water.
NASA typically uses surface air temperatures over land, while using surface water temperatures over oceans. When instead using air temperatures globally, the temperature anomaly could be more than 0.1°C higher.
- Missing data
How are missing data dealt with? To calculate the global mean on maps, NASA uses four zonal regions (90-24ºS, 24-0ºS, 0-24ºN, and 24-90ºN) and fills gaps in a region by the mean over the available data in that region. In datasets, however, missing data are typically ignored. This could make a difference of 0.2°C. Ignoring data for the Arctic alone could make a difference of 0.1°C.
Anthropogenic Global Warming
Remember the Paris Agreement, when politicians pledged to take efforts to ensure that the temperature would not cross 1.5°C above preindustrial? Why did the Paris Agreement not specify a year for preindustrial? Perhaps the idea was that total anthropogenic global warming should not exceed 1.5°C. In other words, the warming that people had already caused by 1750, plus the warming people caused since 1750, plus the warming that is already baked in for the decades to come. The image below illustrates this idea and also shows that we're well above 1.5°C anthropogenic global warming.
|[ click on images to enlarge ]|
recent paper, James Hansen et al. conclude that temperatures also weren't more than 1°C above preindustrial during the previous interglacial, the Eemian, which implies that temperatures haven't been more than 1°C above preindustrial for the entire 200,000 years that modern people, i.e. the species homo sapiens, have existed, and that temperatures have only recently rising to levels more than 1°C above preindustrial. Quite likely, to find temperatures as high as today's, one would have to go back some 3 million years.
Fires over North America, August 2018
Fires can significantly influence temperatures in a number of ways. The images below show how fires boosted carbon dioxide, carbon monoxide and sulfur dioxide levels on August 19, 2018. Carbon dioxide and carbon monoxide both raise temperatures. On the other hand, sulfur dioxide lowers temperature by reflecting sunlight back into space.
The image below illustrates to what extent smoke from fires boosted black carbon in the air over North America on August 23, 2018. Black carbon causes both cooling and warming. Black carbon shades the surface, somewhat cooling the surface of land and water, while it also absorbs heat, thus warming the air above the surface. Furthermore, black carbon causes warming by darkening the surface once it settles down. Studies have calculated that black carbon has a total net global warming effect of more than 1.1 W/m².
Dust and further aerosols
The impact of aerosols such as sulfur dioxide and dust is often overlooked. The image below shows that τ, i.e. light at 550 nm as a measurement of aerosol optical thickness due to dust aerosols, was as high as 4.0641 on June 16, 2018.
|[ goats, from Wikipedia ]|
Most dust appears to originate from the Sahara Desert, which lost its vegetation during the Holocene due to goats, according to this study, as people removed predators such as lions and tigers. As the Sahara lost its vegetation, the surface became more reflective, while dust further made that temperatures didn't rise as much as they otherwise would have.
Deforestation has caused a lot of carbon dioxide to be added during preindustrial times, and there is also the impact of black carbon aerosols, resulting from biomass and fossil fuel burning, which causes some 1.1W/m² warming today and some 0.2W/m² is coming from preindustrial activities.
In conclusion, temperatures would be a lot lower in the absence of human activities, while total anthropogenic global warming over the past few thousand years is much larger than most people think.
The situation is dire and calls for comprehensive and effective action, as described in the Climate Plan.
• NASA - The Northwest is Running Hot and Dry
• NASA GISS (Goddard Institute for Space Studies) Surface Temperature Analysis (GISTEMP)
• NASA - Just Another Day on Aerosol Earth
• How much warming have humans caused?
• How much warmer is it now?
• Climate Plan