The Earth’s Rate of Warming in the 21st Century is 0.2 Celsius per Decade

The most recent Intergovernmental Panel on Climate Change (IPCC) noted that all years in the future will be warmer than those we have experienced in the past. The rate of warming of 0.2 Celsius (0.36 Fahrenheit) per decade means by 2050, mean atmospheric temperatures will be between 0.6 and 0.8 Celsius (from 1 to 1.5 Fahrenheit) warmer than today.

The rate of warming appears to be accelerating regardless of whether we are experiencing the Pacific Ocean oscillating warmth of El Niño or cooling of La Niña events. One of the suspects causing this acceleration can be found in ocean surface temperatures. Since 1901, the ocean’s top 700 metres (2,300 feet) have warmed 0.83 Celsius (1.5 Fahrenheit).

Some scientists point to new maritime shipping pollution standards as the cause. Until 2016 the world’s ships were largely powered by bunker oil, the dirtiest liquid fossil fuel on the planet. Bunker oil when burned emits greenhouse gasses and particulate matter at significantly higher rates than any other fossil fuel.

In 2016, the International Maritime Organization (IMO) committed to tackling the problem. That year, it announced new fuel standards designed to reduce sulphur content by 80%. It also required smokestacks to install scrubbers to remove carbon particulate matter. The changes were to affect 75% of maritime ships that were contributing thousands of tons of sulphur dioxide and 21% of the total amount of black carbon particulate matter entering our atmosphere annually. Those ship tracks of black sooty smoke that were common sights at sea began to vanish. Today the sight of a black-spewing ship smokestack has become rare.

Less carbon and sulphur are now entering the air over our oceans affecting cloud formation and the amount of sunlight reaching water surfaces. Climate modellers have been measuring the additional energy being absorbed by this increased sunlight at +1.2 watts per square metre. Climate modellers have a more ominous warning noting that the latent energy being collected by the ocean as the heat spreads throughout the entire water column will only speed up the phenomenon.

There are numerous other causes for atmospheric and ocean warming as climate scientists have noted. The one pointing to cleaning up ship pollution, however, is the most surprising. Doing the right thing about sulphur and carbon emissions has produced unintended consequences. Clearer air over our oceans is now seen as a global warming trigger.

This begs the question, if we remove pollution from the atmosphere to impact clouds over water, will the same happen over land? The role of clouds in climate change remains a large uncertainty. They are a dominant feature of Earth’s atmosphere, on average, covering 60% of the planet on any given day. When present over land their cooling capacity amounts to approximately 20 watts per square metre. Shape, location, altitude, water content, and particle composition all contribute to cloud dynamics and warming and cooling capacity.

How does land surface warming affect clouds?

• Warmer land surfaces produce fewer clouds which means less sunlight is reflected into space.
• Warmer land surfaces cause clouds to form at higher altitudes.
• Warmer land surfaces reduce cloud cover during the day while causing clouds to form at night trapping any radiated heat (the greenhouse effect).

Clouds produce a feedback loop that has only recently become a subject of study. The European Centre for Medium-Range Weather Forecasts (ERAS), the National Oceanic and Atmospheric Administration (NOAA), NASA and partners including Canada, France and Japan have been conducting extensive research on cloud characteristics and how they regulate energy in the atmosphere and ocean.

To study clouds several satellites have been deployed. In 2006, NASA and the Canadian Space Agency (CSA) launched CloudSat which continues to provide quantitative estimates of water content distribution in the atmosphere. CloudSat studies the role clouds play in heating and cooling. It has been joined by a constellation of additional satellites called the A-Train. These go by the names CALIPSO, Aqua, Aura, OCO-2 and GCOM-W1 and represent an international collaboration of space scientists and agencies. The A-Train constellation has given us a better understanding of cloud composition and its role in atmospheric dynamics. It is also keeping tabs on the condition of Earth’s ozone layer.

Joining CloudSat in 2024, a joint scientific collaboration of ESA and JAXA launched EarthCARE. The satellite’s instrumentation is shedding light on the interactions between clouds, aerosols, and solar radiative forcing. It should enlighten us on the role of these various components within clouds and their cooling effects and contribute to our understanding of the feedback loops from clouds that contribute to climate change.

For some, re-engineering clouds or generating human-engineered ones is seen as a climate mitigation strategy. This form of geoengineering has been proposed by people like Peter Irvine, Professor of Earth Sciences at Harvard University who in 2020 suggested aerosol geoengineering in the high atmosphere could induce clouds to manage climate change. Other proposed geoengineering projects include spraying chemicals into the atmosphere to cause cloud brightening which could lower land and surface ocean temperatures. The problem with geoengineering is that it raises more questions than it provides solutions and is likely a road climate science shouldn’t pursue. We already are seeing unintended consequences from the IMO’s good samaritan policies to reduce bunker oil pollution. The results have been surprising, to say the least.