HIGHLIGHTS
• The extent of global warming after emissions cease for every year starting in 2021 until 2080, and taking into account emission from all known sources, has been prepared in a new study.
• If emissions continue under the current scenario, the two-degree centigrade increase is projected to be reached around the middle of the 21st century.
• If emissions continue under the current scenario, carbon dioxide concentrations in the atmosphere are predicted to approximately double from current levels, and the average global temperature could increase by 3.6 degrees centigrade by 2100. 

The ultimate aim for achieving sustainability is to reduce the rate of global warming. The tribology and lubrication field has become involved in this objective because reducing emissions, and improving productivity and efficiency, can be achieved through the proper use of lubricants.

A significant example is the automotive industry’s move to transition from internal combustion powered vehicles to battery electric vehicles. A previous TLT article¹ discussed a life cycle assessment (LCA) study determining the emissions generated by sedans, SUVs and pickup trucks throughout the continental U.S. during the lifetime of these vehicles. Initially, battery electric vehicles display higher emissions than internal combustion engine vehicles during their production. But once in operation, the equal emissions point is reached in 1.2-1.6 years. As the vehicles are operated over a lifetime, the researchers found that battery electric vehicle sedans have 64% fewer life cycle emissions than their internal combustion engine counterparts.

The Paris Agreement adopted in December 2015 has the objective of limiting global warming to an increase of two degrees centigrade compared to pre-industrial levels. Ideally, the agreement calls for global warming to increase by no more than 1.5 degrees centigrade.

Michelle Dvorak, graduate student in the School of Oceanography at the University of Washington in Seattle, says, “Past research has shown that there is a linear relationship between global warming and carbon dioxide emissions. Future increases in global temperature will be determined to a large extent on what efforts are made to slow down carbon dioxide emissions. This means that global warming will be governed by future and not past emissions.”

But other emissions such as methane, nitrogen oxide and aerosols also must be taken into consideration. Dvorak says, “Aerosols have the short-term effect of causing cooling in the atmosphere by scattering and reflecting sunlight. They also help to promote the formation of clouds that reflect sunlight. If aerosol generation is eliminated, then a global temperature rise will take place for a short period of time followed by a reduction if other emissions including carbon dioxide are stopped.”

One approach for studying future global warming is to predict how temperature will change after a time when all emissions cease. This is known as the geophysical warming commitment.

A new study has now been conducted that predicts the extent of global warming after emissions cease for every year starting in 2021 until 2080 and takes into account emissions from all known sources.

Finite Amplitude Impulse Response (FaIR) Model
Dvorak and her colleagues, including Kyle Armour, associate professor in the department of atmospheric sciences at the University of Washington, used an emissions-based climate model known as the Finite Amplitude Impulse Response (FaIR) Model that has parameters established from observations of global energy budget and temperature trends since the 19th century. She says, “FaIR is a fairly simple model that can be run on a laptop computer. The core of the model is a simple algorithm that solves for the temperature of two boxes, which together are representatives of the climate system. The first box represents the surface climate, including the land, atmosphere and the top 10 meters of the ocean, which warms quickly in response to incoming radiation due to a small combined heat capacity. The second box represents the deep ocean down to a depth of 2,000 meters, which has a much larger heat capacity, and, therefore, adjusts slowly to radiative forcing.”

In conducting the analysis, the researchers followed eight Shared Socioeconomic Pathways (SSPs) that predict different rates of climate change based on social and economic factors in addition to the degree of emissions. Dvorak says, “SSPs are based on the premise that very low emissions occur due to high mitigation, and very high emissions are due to low mitigation.”

Analysis from SSP 2-4.5, which is considered the “middle of the road” scenario where carbon dioxide emissions remain at current levels until the middle of the 21st century but do not reach net zero until 2100, is shown in Figure 1 for when all emissions end at the beginning of 2021. This graph shows global temperature change versus the year until 2100.


Figure 1. Predictions for various global warming scenarios are shown using a “middle of the road” scenario known as SSP 2-4.5 from the year 2021 until 2100. The orange line represents how global warming will increase over time. The dotted line projects what will happen if just carbon dioxide emissions cease in 2021. If all emissions cease in 2021 as shown in the dotted line curve, the global temperature will slightly increase and then gradually decrease to one degree centigrade above preindustrial temperatures by 2100. Figure courtesy of the University of Washington.

The orange line represents the current scenario where there is no cessation of emissions. This analysis indicates that the two-degree centigrade increase will be reached around the middle of the 21st century. If just carbon dioxide emissions cease in 2021 (the dotted line), then the temperature will climb to just over one degree centigrade and remain flat for the rest of the century. But if all emissions are eliminated (the dashed line), the model predicts a peak in global temperature to almost 1.5 C, followed by a slow cooling to one degree centigrade above preindustrial temperatures by 2100.

Based on use of the FaIR model, the researchers determined there is a 43% probability of a peak warming greater than 1.5 degrees centigrade if all emissions ceased in 2021. This probability increases to 66% if all emissions cease by 2029. The same model forecasts that the probability of a two-degree centigrade increase in temperature is 2% if all emissions ceased in 2021. This figure increases to 66% if all emission do not cease until 2057.

Dvorak says, “We suspect that global warming is probably following SSP 3-7.0, which forecasts a larger increase in global temperature than SSP 2-4.5. This new pathway projects a steady rise in temperature and emissions. By 2100, carbon dioxide concentrations are predicted to approximately double current levels, and the average global temperature could increase by 3.6 degrees centigrade.”

The researchers are now evaluating what happened to past climate in the Pliocene period when carbon dioxide levels were last comparable to levels seen today. Dvorak says, “We would like to determine how the high levels of carbon dioxide in the atmosphere three million years ago contributed to the climate changing during this period, and how similar those changes were to projected global warming.”

Additional information can be found in a recent article² or by contacting Dvorak at mtdvorak@u.washington.edu.

REFERENCES
1. Canter, N. (2022), “Effect of electrification of light-duty vehicles on carbon dioxide emissions,” TLT, 78 (6), pp. 28-29. Available here.
2. Dvorak, M., Armour, K., Frierson, D., Proistosescu, C., Baker, M. and Smith, C. (2022), “Estimating the timing of geophysical commitment to 1.5 and 2.0 C of global warming,” Nature Climate Change, 12 (6), pp. 547-552.