How to explain global warming?

The question of Attribution

You learned about the evidence that proves anthropogenic climate change is taking place. Now, let’s talk about how we explain the phenomena of global warming.

Previously, you viewed this figure from the IPCC’s assessment report, showing various factors that contribute to climate change. The next slide will include further detail about each forcing component.

This figure is also from the IPCC’s assessment report. LOSU means ‘level of scientific understanding’. In this figure, two different forcing components are shown; anthropogenic and natural forcings. It is important to remember that not only anthropogenic forcings, natural forcings also drive climate change. For example, glacial/Interglacial cycles we observed from the ice core samples earlier this semester that recorded atmospheric conditions over last 450,000 years are clearly caused by natural forcings as we, homo sapiens, did not exist that time!

In this figure, each radiative forcing is associated with a value (watts per square meter) quantifying how much each forcing contributes to climate change. Some forcings have a negative number (contribute to cooling), whereas others have a positive number (contribute to warming). The total net forcing is currently a positive value. Thus, the climate trend is currently warming.

IPCC report

As shown in the previous figure, natural forcing can change climate. The dominant energy source to change Earth’s climate, the sun, also varies its energy emission. This figure shows natural changes in solar irradiance from 1874 to 1988. Solar irradiance is the amount of energy per unit area received from the Sun. In recent decades, solar activity has been measured by satellites, while before it was estimated using a proxy variation. Without satellite observation, energy differences were too small to detect.

Solar irradiance is higher during a period called “solar maximum”, which appears almost every 11 years. During a solar maximum, interesting features that appears on the Sun’s surface…

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Solar luminositySunspot cycle (~11 year period, ~0.1% change in radiation output)

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…are sunspots! Sunspots are relatively dark areas on the radiating surface of the Sun, where intense magnetic activity inhibits convection and cools the photosphere. Luminosity is the total amount of energy emitted by the Sun.

To summarize, more sunspot appears during a period of solar maximum, when the Sun presents more intense magnetic activity (therefore higher luminosity).

Although solar irradiance was only recently measured by satellite, sunspots have been observed for a very long time! The first such recording was made by Galileo Galilei in the 17th century when he created the first telescope. In addition, there are well documented historical records that captured solar activity by Chinese astronomers. All records combined confirm the 11 year cycle that coincide with a solar maximum.

Further reading:https://solarscience.msfc.nasa.gov/SunspotCycle.shtml

http://sxi.ngdc.noaa.gov/sxi_greatest.html

During periods when there is active solar activity (more sunspots), scientists have detected lower cosmic ray intensity on Earth; though a fraction of those rays, short wave cosmic rays (high energy), are harmful to living organisms. The inverse relationship between the number of sunspots and intensity of cosmic rays shown in this figure explain that electro magnetic energy emitted by the Sun actually shields such harmful cosmic rays.

The other important natural forcing that causes a climate response is volcanic eruptions. In this figure from the IPCC report, you see changes in radiative forcingsbetween 1850 and 2000. The blue solid line shows strong negative forcing that happens sporadically and coincides with significant historic volcanic eruptions. When a volcanic eruptions is significantly large, it will eject large amounts of sulfur dioxide into the stratosphere. This sulfur dioxide will eventually break down into an aerosol form and, the stratosphere being very stable, will remain in the stratosphere for up to few years; blocking sunlight that reaches the Earth’s surface, and thereby decreasing temperature. Although this only has a short term effect, up to few years, temperature drops can be significant and noticeable, and such periods are called “volcanic winter”.

In this figure, abrupt decreases in volcanic radiative forcing are associated with Krakatowa in 1883 (caused 5 year volcanic winter), Santa Maria in 1902, Mt. Peleein 1902 (caused casualties of 33000 people), Mt. St. Helens in 1980, El Chichon in 1982 (caused casualties of 3500 people), and Mt. Pinatubo in 1991 (largest stratospheric disturbance since Krakatowa).

Hansen et al. 2005

This figure is a summary of climate forcings along with global temperatures from 1880 to 2000 published in the journal of Science.

AttributionObserved changes consistent with: þ expected responses

to forcingsQ inconsistent with

alternative explanations (part of a natural cycle)

Observations

All forcing

Solar+volcanic

IPCC 4rth assessment report

Climate models are important tools for attributing and understanding climate change. Understanding observed changes is based on our best understanding of climate physics, as contained in simple to complex climate models. For the 4th IPCC assessment report, you see observed global and annual mean temperatures in black over the 20th century compared to the temperature simulated by a wide range of climate models. On the top, in red, individual model simulations are driven by external influences of both anthropogenic and natural forcings, including changes in greenhouse gases, aerosols, solar radiation and volcanic eruptions. These observations rarely leave the range of those predicted by model simulations.

The lower figure compares the observed global and annual mean temperature (in black) to the individual model simulations driven by only natural forcings including changes in solar radiation and by volcanic eruption (in blue).

To summarize, natural forcings cannot explain the temperature increase in recent years without anthropogenic forcing!

The IPCC synthesis report for policymakers clearly states that the climate system is warming and is largely influenced by anthropogenic causes.

IPCC Special Report on Global Warming of 1.5 ºC (2019)

And in 2019, the IPCC published a special report on Global Warming of 1.5 ºC. Here is a quote from their statement providing quantitative values on how much human activities have contributed to global warming.

“Human activities are estimated to have caused approximately 1.0�C of global warming5 above pre-industrial levels, with a likely range of 0.8�C to 1.2�C. Global warming is likely to reach 1.5�C between 2030 and 2052 if it continues to increase at the current rate. (high confidence) ”

SPM Summary for Policymakers, p4

IPCC Special Report on Global Warming of 1.5 ºC (2019)