Peak Carbon Dioxide Emissions and Peak Carbon Dioxide Concentration

David P. Turner / December 2, 2020

A remarkable speculation is now circulating in the cybersphere to the effect that global emissions of carbon dioxide (CO2) from fossil fuel combustion may have peaked in 2019.  Considering that recent formal projections generally indicate increasing emissions through 2030 or longer, this assertion is striking.  It matters because CO2 emissions determine the growth in the atmospheric CO2 concentration, which in turn influences the magnitude of global warming.

The atmospheric CO2 concentration is currently 415 ppm (up from a preindustrial value of around 280 ppm) and is rising at a rate of 2-3 ppm per year.  The consensus among climate scientists is that rapid greenhouse-gas-driven climate change will be harmful to the human enterprise on Earth.  It would be good news indeed if CO2 emissions were on the way down.

Estimates for annual global CO2 emissions are produced by assembling data on consumption of coal, oil, and natural gas, as well as data on production of cement, which also releases CO2 (the sum is termed Fossil Fuel & Industry emissions).  Deforestation is another significant anthropogenic source of CO2, but it is not considered in this blog post except to say that reducing deforestation will further reduce total CO2 emissions.

The suggestion that we are passing peak fossil fuel emissions is based on several observations.

1.  The rate of increase in global emissions has been low in recent years, averaging less than 1% per year for 2010-2018.  CO2 emissions are falling in the US and the EU, and the annual rate of increase in emissions is declining in India

Covid-19 related reductions in global fossil fuel consumption for 2020 will be 7% or more.  Emissions will likely rebound to some degree as the pandemic recedes but perhaps not fully.

2.  Peak global coal use likely occurred in 2013.  Aging coal powered electricity plants in the U.S. are often replaced with plants powered by natural gas (more efficient that coal) or renewable energy.  Some coal plants are being prematurely retired.  A gradual phase out in global coal consumption will be driven by the price advantage of renewable energy, impacts of coal emissions on human health, and the reluctance of insurance companies to cover new coal power plant construction.

3.  Peak oil use may have occurred in 2019.  Global demand in 2020 will fall about 10% because of Covid-19.  Structural changes such as reduced commuting and business-related flying mean that some of the demand reductions will be persistent.  Vehicles powered by electricity and hydrogen rather than gasoline are on the ascendancy, sparked in part by governmental mandates to phase in zero emissions vehicles.

4.  Even a near term peak in natural gas consumption is being discussed.  Again, the price advantage of renewable sources will increasingly weigh against fossil-fuel-based power plants.  Ramped up production of renewable natural gas could substitute for fossil natural gas in some applications.

Surprisingly, it appears likely that a long-term decline in total fossil fuel use will be driven more by lack of demand than lack of supply.

Emissions from cement manufacturing are still climbing and amount to about 4% of total fossil fuel emissions.  However, a recent study suggests that the CO2 uptake from slow weathering of aging cement around the world is providing a large offset (more than half) to current cement manufacturing emissions.  Innovative uses of wood and geopolymers can potentially replace cement in many construction applications.

The election of Biden to the U.S. presidency is also relevant.  Biden’s leadership will return the U.S. (largest cumulative CO2 emissions on the planet) to the international fold with respect to climate change mitigation.  President Xi Jinping of China (largest CO2 emitter on the planet) has also displayed leadership (in words if not deeds) on the climate change issue.  A revitalized collaboration between the U.S. and China on climate change mitigation could push the needle on global emissions reduction.

Currently about half of fossil fuel CO2 emissions remain in the atmosphere, with the remainder sequestered on the land (e.g. in vegetation and soil) and in the ocean.  Once fossil fuel emissions begin decreasing and fall by half − and assuming the net effect of increasing CO2 and climate warming is still substantial carbon uptake by the land and ocean − the atmospheric CO2 concentration will peak and begin to decrease.  The year of peak CO2 concentration could be as early as 2040 (see carbon cycle projection tool below).

There is of course plenty that might go wrong.  The net effect on the land and ocean sequestration just referred to could be a decline in carbon uptake.  On land, carbon sources such as permafrost melting and forest fires will be stimulated by climate warming.  In the ocean, warming will intensify stratification, thereby reducing carbon removal to the ocean interior. 

On the other hand, land sequestration is increasing now and could continue to do so in response to CO2 enhancement of photosynthesis and plant water use efficiency.  Policy driven increases in the land carbon sink (e.g. more reforestation and afforestation) are also possible.  The ocean carbon sink is likewise increasing now, continuing an upward trend over the last 20 years.

Whatever specific years do turn out to be peak CO2 emissions and peak CO2 concentration, they will be remembered as historic hallmarks in humanity’s effort to address an existential threat of its own making.

Recommended:  Interactive CO2 Emissions and Concentration Projection Tool.

Peak Human Population and the Global Environment

David P. Turner / September 11, 2020


A key pursuit in the field of Earth System Science is measuring and monitoring global scale structures and processes.  These measurements have led to the concept of the “Great Acceleration”, a name given to the period since around 1950 during which many global scale attributes related to the human enterprise (the technosphere) began rising in an exponential fashion.  The increase in global population is the iconic example.

Intuitively, it seems unlikely that this level of population increase and associated resource consumption could continue indefinitely on a finite planet.  Practically speaking, problems have begun to arise both with resource shortages and environmental degradation from excess waste production (e.g. global warming and ocean acidification from massive fossil fuel combustion).

Humanity clearly must transition to a more sustainable relationship with the rest of the Earth system.  The way forward lies in bending those exponentially rising Great Acceleration curves for population and resources use, hitting the peaks, and engineering declines.

As noted by ecologists long ago, total resource use (Impact) is a function of the number of people (Population), their per capita use (Affluence), and the efficiency with which raw resources are converted to useful products (Technology).

Resource use per person obviously varies tremendously, hinting at the special responsibility of the more developed countries to limit population growth (the net effect of births minus deaths and immigration minus emigration).  But all humans consume natural resources.  Thus, the high projected population growth rates in less developed countries must also be brought down.  The sooner global population peaks, the less natural capital (e.g. biodiversity) will be degraded, the less likely that competition for resources will lead to human conflict, and the less likely that climate change will trigger tipping points in the Earth system that precipitate extreme impacts on humans.

Past, Present, and Future Global Population

The global population size doubled between 1927 and 1974 and has nearly doubled again since 1979.  It is now 7.8 billion.

However, the rate of annual global population growth has fallen in recent decades (from > 2% per year to 1.05% per year), mostly associated with a decreasing trend in fertility (children born per woman during her reproductive lifetime).

Family planning programs by governmental and nongovernment organizations have significantly impacted the trend towards lower fertility rates.

Projections by demographers of peak global population range widely.  The median estimate from the United Nations Population Division is for a population of 10.9 billion in 2100.  Most of the increase from the present is in Africa.

However, recent research points toward lower values, possibly a peak of 9.7 billion around 2064 and a decline to 8.8 billion by 2100. 

Factors Influencing Demographic Projections

Projections of peak global population have significant policy implications.  Relatively low estimates may have the effect that national commitments to stabilize population are downgraded and that overhyped media accounts of depopulation sap political will to continue family planning programs.  Relatively high estimates for peak global population foster the impression that humanity it doomed to an overcrowded and overheated planet, hence favoring lifeboat ethics.

Despite the critical implications of their results, the models used to predict peak population are very sensitive to the assumptions made about trends in fertility. 

The recent lower estimates for peak global population rely on continued or increasing reductions in fertility in the high fertility countries.  But demographers in the past have sometimes overestimated declines in fertility, and may be doing so now as well.  Historic trends of declining fertility have stalled in some high fertility countries, possibly related to falling support for family planning.  The Catholic Church still formally prohibits artificial birth control. 

Nevertheless, several emerging trends may support lower projected peaks in global population. 

One is that efforts to shift cultural norms favoring large family size increasingly include family planning messaging in popular media (e.g. serial dramas), which are having significant success with both genders

Another is that the incidence of unplanned pregnancy is declining globally (1990-2014), probably as a function of improving access to family planning resources.

The Covid-19 pandemic could push birth rates down (at least in the more developed countries) because financial insecurity will dispose women in developed and developing countries to postpone or forgo having children.  

Mortality rates may also be higher than expected.  Life expectancy has generally increased in recent decades throughout the world.  Much of that increase is associated with reduced child mortality but increasing longevity is also a factor.  However, life expectancy in the U.S. went down from 2014 to 2017 because of increasing fatal drug overdoses and suicides.  Climate change is expected to bring an increase in extreme weather events causing mortality directly (as in flooding), and indirectly by way of impacts on agriculture and possibly the incidence of war.

Implications Beyond Absolute Population Size

A leading concern about a rapid peak and then decline in national populations is the associated increase in the ratio of older retired people to younger working people.  As the population ages, the number of active workers available to support each elderly person tends to decline.  Hence, taxes may have to be increased to provide income and health care to the elderly.  Various mitigating factors include the improving health of elderly people, significant intergenerational transfers of wealth, increases in labor force participation by the elderly, and volunteer efforts by the elderly.

Also, a larger proportion of elderly people generally means decreased per capita demand for resources and a more peaceful society.

A decline in the number of children per family can have many beneficial side effects including: 1) more resources (parental attention and ability to finance education) per child, 2) improved quality of life for parents (less stress and more free time), and 3) rising per capita income.


The sooner global population peaks and begins to decline, the greater the possibilities for achieving global sustainability.  Since about 40% of pregnancies globally are still unplanned, a primary tool for insuring children are born into a welcoming and opportunity-rich environment is continued and improved provision of family planning support in both the developing and developed world.  More political will and contributions to NGOs are needed.  At this point in human history, the local and global challenges (environmental, economic, and social) that arise from a stable or declining population are likely more manageable than those arising from high rates of population growth.