OPED: Climate models and global warming

RBC | In this article, I summarize scientific consensus on the state of the global climate and explain the basis for climate predictions. Climate projections are based on computer models, and few people in the general population understand how the models work. If there’s been a failure in climate science, perhaps it is this: climate science understands the trends in the global climate, but we haven’t done a very good job explaining the models by which we came to that understanding.

Two major reports published in the last few months summarize scientific consensus on climate: the updated Intergovernmental Panel (IPCC) report analyzes the effects of a 1.5 degree Celsius  (about 2.7 degree Fahrenheit) rise in average global temperature, and the Fourth National Climate Assessment from the U.S. Global Change Research Program analyzes effects on ecosystems, agriculture, infrastructure and the economy.

Who writes these reports? What is their purpose? Why can we believe them?

Last week I described the process of science by which we study climate and the effects of climate change. Many thousands of independent scientists, working in different places all over the world, study questions of particular interest to them. Some study ant colonies in the rainforest. Others study cichlid fishes in the great lakes of Africa. Others study ice volume on Greenland. The climate research community comprises thousands of scientists asking questions about how the planet works. Their findings are published only after rigorous review by experts in the field, and their findings are accepted only after they have been confirmed by other scientists.

Every four years a panel of climate scientists and policy experts in the United States, mandated by Act of Congress, review the latest reports from scientists in the field and update their recommendations for policy makers in Congress and the executive branch. Their findings are published as the “National Climate Assessment.” Also every four to five years the Intergovernmental Panel on Climate Change (IPCC) convenes for the same purpose but with a wider audience. The IPCC includes more than 400 climate scientists and policy experts from all over the world. They review the most recent research and updated climate models, and their reports provide guidance for governments all around the world.

– The latest reports from IPCC and the National Climate Assessment warn of impending severe disruption to ecosystems, to agriculture and to infrastructure.

The most recent IPCC summary, released in October 2018, is based on review of more than 6,000 published research studies by many hundreds of climate scientists all over the globe. It finds that the climate is changing more rapidly than anticipated and that even a rise of 1.5 degrees Centigrade above pre-industrial average global temperature will very likely result in catastrophic disruption to ecosystems, water supply, and agricultural productivity. A change of that magnitude also very likely will result in more extreme weather events (stronger hurricanes, more intense rainfall, etc.), increasing extreme heat events and heat deaths, larger and more intense wildfires, and emerging infectious diseases, among other potential catastrophes. Previous assessments have used 2 degrees Celsius as the benchmark for projections; 1.5 degree Celsius warming is our near future. We have about 12 years, given current trends, to re-align the world economy to net zero carbon emissions.

The Fourth National Climate Assessment, published in November 2018, presents the same picture with specific reference to the United States. Following are quotations with summary findings from the Assessment. The particulars, with links to the primary research, are available online (U.S. Global Climate Research Program, 2018).

– Climate change creates new risks and exacerbates existing vulnerabilities in communities across the United States, presenting growing challenges to human health and safety, quality of life, and the rate of economic growth.

– Without substantial and sustained global mitigation and regional adaptation efforts, climate change is expected to cause growing losses to American infrastructure and property and impede the rate of economic growth over this century.

– Communities, governments, and businesses are working to reduce risks from and costs associated with climate change by taking action to lower greenhouse gas emissions and implement adaptation strategies. While mitigation and adaptation efforts have expanded substantially in the last four years, they do not yet approach the scale considered necessary to avoid substantial damages to the economy, environment, and human health over the coming decades.

– The quality and quantity of water available for use by people and ecosystems across the country are being affected by climate change, increasing risks and costs to agriculture, energy production, industry, recreation, and the environment.

– Impacts from climate change on extreme weather and climate-related events, air quality, and the transmission of disease through insects and pests, food, and water increasingly threaten the health and well-being of the American people, particularly populations that are already vulnerable.

– Ecosystems and the benefits they provide to society are being altered by climate change, and these impacts are projected to continue. Without substantial and sustained reductions in global greenhouse gas emissions, transformative impacts on some ecosystems will occur; some coral reef and sea ice ecosystems are already experiencing such transformational changes.

– Rising temperatures, extreme heat, drought, wildfire on rangelands, and heavy downpours are expected to increasingly disrupt agricultural productivity in the United States. Expected increases in challenges to livestock health, declines in crop yields and quality, and changes in extreme events in the United States and abroad threaten rural livelihoods, sustainable food security, and price stability.

– Our nation’s aging and deteriorating infrastructure is further stressed by increases in heavy precipitation events, coastal flooding, heat, wildfires and other extreme events, as well as changes to average precipitation and temperature. Without adaptation, climate change will continue to degrade infrastructure performance over the rest of the century, with the potential for cascading impacts that threaten our economy, national security, essential services and health and well-being.

– Coastal communities and the ecosystems that support them are increasingly threatened by the impacts of climate change. Without significant reductions in global greenhouse gas emissions and regional adaptation measures, many coastal regions will be transformed by the latter part of this century, with impacts affecting other regions and sectors. Even in a future with lower greenhouse gas emissions, many communities are expected to suffer financial impacts as chronic high-tide flooding leads to higher costs and lower property values.

– Outdoor recreation, tourist economies, and quality of life are reliant on benefits provided by our natural environment that will be degraded by the impacts of climate change in many ways.

– The assessment notes that people living in poverty and, especially, native peoples are disproportionately affected by climate change because they live in more vulnerable areas, rely more heavily on ecosystem services, and lack the resources to adapt.

As summarized by the IPCC and the National Climate Assessment, there is no doubt among scientists that climate change is real. It is human-caused. And it threatens human existence as well as the planet’s web of life.

– Projections of future climate are based on climate models that analyze and extrapolate measurements from field research all over the globe.

How is it possible to predict future climate? Why do IPCC and NCA say we’ve got about 12 years to re-engineer the world economy?

Measuring global temperature and precipitation is straightforward; put thermometers and rain gauges all over the planet and record those measurements year to year over decades and centuries. Comparing present climate to the distant past requires more sophisticated tools. Tree rings take us back hundreds or thousands of years. Ice cores, which record year to year accumulation of snowpack and the gases trapped in the snow, can take us back hundreds of thousands of years. Lake and ocean sediments with layers of foraminifera push the record back hundreds of thousands to millions of years. And other proxies can take us back even further. Such studies have been replicated all over the planet, and they show that the present climate is changing faster than at any time in the historic or geologic record (Royal Society, 2018).

– OK, but how do we know that warming isn’t just part of a natural cycle? And how can you predict the climate into the future? We still aren’t very good at knowing whether or not it will snow next week. Here’s where you have to understand the climate models.

Climate models are computer programs that replicate temperature and precipitation (and other climate data) in mathematical equations. Climate models, like other scientific models, are then tested by real world experiment and observation.

First a word on the word, “model.” Scientists often borrow terms from the general vocabulary but assign them specific meaning. And often times that scientific meaning may not correspond to the general usage of the term. “Model,” in science, refers to a set of mathematical equations that capture the behavior of a natural system. A good model accurately reproduces how nature behaves and allows scientists to predict the outcome of experiments. That’s the test of the model: do the results of actual experiments agree with the model’s predictions? A good model passes all the experimental tests thrown at it. Newton’s model of gravity, for example, the formula above accurately reproduces the measured gravitational forces in our solar system and enables us to calculate the rocket forces needed to send spacecraft to the moon and to the distant planets. Another example, the logistic equation (below) accurately models the growth of populations, be it a population of E.coli bacteria in a test tube or a population of caribou on an island in the Bering Sea.

The rate of growth of a population of size N depends on the unrestricted growth rate r and the carrying capacity of the environment K .

Modern science builds models from data then tests those models by experiment and observation. Climate models are scientific models under the definition above. The equations are more complicated because earth’s climate system includes many more variables than just the handful of variables in Newton’s laws or the logistic model. Climate models include contributions from incident solar radiation, earth’s rotation, heat transport by convection, the heat capacity of ocean water, reflectivity of ice and snow, atmospheric CO2 concentration, cloud cover and more. Climate models require many hours of computation on supercomputers. We have the computers. We have the models. And they work. Like other scientific models, climate models have been tested rigorously against observation and experiment.

How on earth do you do that? Test the predictions of a climate model against the actual results of an experiment? You can’t experiment with planet earth.

Well, it turns out you can. Here’s the trick: reset the clock in your computer. Tell the computer it’s the year 1900. Give the computer all the climate data it needs from the records for that year. Solar radiation, snow cover, jet stream track, concentration of greenhouse gases in the atmosphere, etc. Then let the model run, updating from year to year 1901, 1902, ’03, ’04 . . . Output the model’s calculated temperature and precipitation for those years. Compare with the actual weather-station records. See if they agree.

The best available models are pretty much spot-on. They successfully retrodict known climate. Temperature and precipitation graphs generated by the models for the 20th century track the actual records. The models also have successfully reproduced the effects of unanticipated events such as the eruption of Mt. Pinatubo. And they have successfully predicted otherwise unanticipated patterns of global warming such as the dramatic increase in Arctic temperatures. (See Skeptical Science, 2018).  So we have considerable confidence that they can predict future climate. And it’s those models that provide compelling evidence it is human activity, particularly our use of fossil fuels, that drives current global warming. Run the models without oil and gas and coal combustion and you get steady global temperatures. Run the models with the addition of fossil fuel combustion and the models reproduce the rising temperature trend that we actually see.

The climate is warming rapidly. That is absolutely clear in the records. And it’s warming because of human activity, particularly the addition of greenhouse gases to the atmosphere. That is absolutely clear in the models and in the observations testing those models.


Carbon Brief. Climate modelling. https://www.carbonbrief.org/category/science/climate-modelling

Intergovermental Panel on Climate Change. October 2018. Global warming of . https://report.ipcc.ch/sr15/pdf/sr15_spm_final.pdf

NASA Climate Kids. How do climate models work? https://climatekids.nasa.gov/climate-model/

National Oceanographic and Atmospheric Administration. Climate models. https://www.climate.gov/maps-data/primer/climate-models

Royal Society. 2018. Climate is always changing. Why is climate change of concern now?


Skeptical Science. 2018. How reliable are climate models? https://www.skepticalscience.com/climate-models.htm

U.S. Global Change Research Program.  November 2018. Fourth National Climate Assessment.  https://nca2018.globalchange.gov/

By BOB DORSETT, M.D. | Special to the Herald Times