Air Pollution's Surprising Role In Ice Age Formation

The Earth's climate has fluctuated between periods of warmth and cold over the last 2 million years, with global average surface temperatures changing by as much as 5°C. The colder periods, known as ice ages, have typically lasted between 80,000 and 100,000 years, while the warmer periods in between have been much shorter, lasting only about 10,000 years. The last ice age ended around 14,000 years ago, and we are currently in an interglacial period. While air pollution from human activity is contributing to global warming, it is not clear whether it could trigger an ice age.

The role of CO2 as a control knob for the Earth's climate

Carbon dioxide (CO2) is the most important climate-relevant greenhouse gas in Earth's atmosphere. CO2 acts as a thermostat in regulating the temperature of Earth. It is a non-condensing greenhouse gas, which provides the core support for the terrestrial greenhouse effect. Non-condensing greenhouse gases, which account for 25% of the total terrestrial greenhouse effect, serve to provide the stable temperature structure that sustains the current levels of atmospheric water vapour and clouds via feedback processes that account for the remaining 75% of the greenhouse effect. Without the radiative forcing supplied by CO2 and the other non-condensing greenhouse gases, the terrestrial greenhouse would collapse, plunging the global climate into an icebound state.

Water vapour and clouds are the major contributors to Earth's greenhouse effect. However, a new atmosphere-ocean climate modelling study shows that the planet's temperature ultimately depends on the atmospheric level of carbon dioxide. The study, conducted by Andrew Lacis and colleagues at NASA's Goddard Institute for Space Studies (GISS) in New York, examined the nature of Earth's greenhouse effect and clarified the role that greenhouse gases and clouds play in absorbing outgoing infrared radiation. Notably, the team identified non-condensing greenhouse gases — such as carbon dioxide, methane, nitrous oxide, ozone, and chlorofluorocarbons — as providing the core support for the terrestrial greenhouse effect.

CO2 is a significant greenhouse gas. It was believed that initial climatic changes resulting from orbital variations were somehow affecting the composition of the atmosphere, to the extent that a lowering of carbon dioxide concentrations was increasing global cooling. The last Ice Age or glacial period on Earth ended roughly 14,000 years ago. At that time, much of Northern Europe and North America lay under huge ice sheets that today remain only in Greenland. At present, we may be coming to the end of the latest warmer interglacial period, although mankind's alteration of the atmosphere through greenhouse gas pollution makes predicting the long-term future of our global climate difficult.

The amount of carbon dioxide in the atmosphere — the main planet-warming greenhouse gas responsible for today's climate change — has risen too, by nearly 50%. The speed of climatic change today is also more or less unprecedented. The amount of CO2 that humans have added over just the last hundred years is comparable to the amount that was added over 100 centuries after the last ice age. In other words, in the modern day, atmospheric carbon has risen about 100 times faster than when humans emerged from the last ice age. That difference is part of why current climate change is so alarming.

The impact of human activity on the next ice age

Human activity has already impacted the current interglacial period, known as the Holocene, and will continue to do so in the future. The Holocene is an interglacial period within the Quaternary Ice Age, which began around 2.58 million years ago. The Holocene has been characterised by warm and cold episodes, with the last cold episode, the Last Glacial Period, ending around 10,000 years ago.

Human activity, particularly the burning of fossil fuels, has led to an increase in atmospheric carbon dioxide (CO2), a potent greenhouse gas. This has resulted in global warming, with temperatures rising by about 6° Celsius (around 11° Fahrenheit) since the last ice age. The accumulation of anthropogenic greenhouse gases is projected to delay the next glacial period by thousands of years.

The current warming caused by human activity differs from the natural warming between ice ages. Usually, changes in the Earth's rotation kick off the warming by increasing the sunlight reaching icy parts of the Earth. As the ice melts, the Earth becomes less reflective, retaining more of the sun's heat. This warming leads to an increase in atmospheric CO2 as it moves from the ocean into the atmosphere, further driving warming.

However, human activity has reversed this process by burning fossil fuels and releasing large amounts of CO2 into the atmosphere, causing warming. The speed of current climate change is also unprecedented. The amount of CO2 released over the last hundred years is comparable to the amount added over 100 centuries during the transition from the last ice age. This rapid change has left ecosystems with much less time to adapt to warming.

Human activity has also impacted the air we breathe. Air pollution, caused mainly by energy use and production, is now the world's fourth-largest risk factor for early death. Outdoor air pollution is responsible for millions of deaths each year, with vulnerable populations such as children, senior citizens, and those with pre-existing health conditions being disproportionately affected.

In summary, human activity, particularly the burning of fossil fuels, has led to increased atmospheric CO2 levels, resulting in global warming and delayed the next ice age. This has disrupted natural climate cycles, impacting ecosystems and human societies that have adapted to the current climate. Additionally, air pollution caused by human activities has negatively affected human health, particularly in vulnerable communities.

The relationship between CO2 and temperature during ice ages

During ice ages, global temperatures decrease, and this is associated with a drop in atmospheric CO2 concentrations. While the specific mechanisms are still being studied, several feedback loops amplify the initial orbital variations and play a crucial role in the development of cold conditions. One key feedback loop involves the solubility of CO2 in water. As oceans cool, they can absorb more CO2, leading to reduced atmospheric CO2 levels. Conversely, during interglacial periods, warmer ocean temperatures result in lower CO2 solubility, leading to increased atmospheric CO2.

Another factor contributing to lower CO2 levels during ice ages is the increased presence of sea ice and glaciers, which reflect more sunlight back into space, further cooling the planet. Additionally, ice sheets and glaciers grind rocks into dust, providing nutrients to ocean life and boosting carbon uptake by the deep ocean. This further reduces atmospheric CO2 concentrations.

It is important to note that while CO2 concentrations and temperature are closely linked during ice ages, the relationship is not always straightforward. In some cases, CO2 lags behind temperature changes by a few hundred years, indicating that it amplifies the initial orbital-driven temperature changes rather than being the primary driver.

Overall, the relationship between CO2 and temperature during ice ages is characterised by complex feedback loops and interactions between various factors, including orbital variations, ocean temperatures, and atmospheric composition.

The influence of orbital variations on ice ages

The term 'Milankovitch cycles' describes the collective effects of changes in the Earth's movements on its climate over thousands of years. The term was coined after Serbian geophysicist and astronomer Milutin Milanković, who, in the 1920s, built on James Croll's earlier hypothesis that variations in eccentricity, axial tilt, and precession combined to result in cyclical variations in the intra-annual and latitudinal distribution of solar radiation at the Earth's surface. This orbital forcing strongly influences the Earth's climatic patterns.

The Earth's rotation around its axis and revolution around the Sun evolve over time due to gravitational interactions with other bodies in the Solar System. The Earth's orbit varies between nearly circular and mildly elliptical. When the orbit is more elongated, there is more variation in the distance between the Earth and the Sun, and in the amount of solar radiation, at different times of the year. In addition, the rotational tilt of the Earth (its obliquity) changes slightly. A greater tilt makes the seasons more extreme. Finally, the direction in which the Earth's axis of rotation points relative to the fixed stars changes (axial precession), while the Earth's elliptical orbit around the Sun rotates (apsidal precession). The combined effect of precession with eccentricity is that proximity to the Sun occurs during different astronomical seasons.

Orbital eccentricity has a dominant period of 405,000 years. Its geometric or logarithmic mean is 0.0019. The present eccentricity is 0.0167 and decreasing. Eccentricity varies primarily due to the gravitational pull of Jupiter and Saturn. The semi-major axis of the orbital ellipse remains unchanged. When Earth's orbit becomes more eccentric, the semi-minor axis shortens. This increases the magnitude of seasonal changes.

The Earth's axial tilt with respect to the orbital plane (the obliquity of the ecliptic) varies between 22.1° and 24.5° over a cycle of about 41,000 years. The current tilt is 23.44°. Increased tilt increases the amplitude of the seasonal cycle in insolation, providing more solar radiation in each hemisphere's summer and less in winter. However, these effects are not uniform everywhere on the Earth's surface. Increased tilt increases the total annual solar radiation at higher latitudes and decreases the total closer to the equator.

Axial precession has a period of about 25,700 years. This motion means that eventually, Polaris will no longer be the North Star. Axial precession is caused by the tidal forces exerted by the Sun and the Moon on the rotating Earth.

The orbital ellipse itself precesses in space in an irregular fashion, completing a full cycle in about 112,000 years relative to the fixed stars. Apsidal precession occurs in the plane of the ecliptic and alters the orientation of the Earth's orbit relative to the ecliptic. Apsidal precession combines with the 25,700-year cycle of axial precession to vary the position in the year that the Earth reaches perihelion. According to a 1965 source, the average value over the last 300,000 years was 23,000 years, varying between 20,800 and 29,000 years.

The inclination of Earth's orbit drifts up and down relative to its present orbit. This three-dimensional movement is known as 'precession of the ecliptic' or 'planetary precession'. Earth's current inclination relative to the invariable plane is 1.57°. When measured independently of Earth's orbit, but relative to the invariable plane, precession has a period of about 100,000 years. This period is very similar to the 100,000-year eccentricity period. Both periods closely match the 100,000-year pattern of glacial events.

Of all the orbital cycles, Milankovitch believed that obliquity had the greatest effect on climate, and that it did so by varying the summer insolation in northern high latitudes. Therefore, he deduced a 41,000-year period for ice ages. However, subsequent research has shown that ice age cycles of the Quaternary glaciation over the last million years have been at a period of 100,000 years, which matches the eccentricity cycle.

The average length of glacial periods has changed over time, from cycles of roughly 40,000 years that were more closely aligned with changes in obliquity, to cycles of roughly 100,000 years, coinciding with changes in the eccentricity of our planet's orbit. The transition between 40,000- and 100,000-year cycles last occurred in the mid-Pleistocene.

The feedback loop between ice sheets and atmospheric CO2

Ice sheets play a critical role in the feedback loop between ice sheets and atmospheric CO2. Ice sheets reflect more incoming solar radiation, which has a cooling effect on the planet. This is known as the albedo effect. As ice sheets melt due to rising temperatures, more incoming solar radiation is absorbed by the Earth, leading to further warming. This positive feedback loop amplifies the effects of climate change.

The relationship between ice sheets and atmospheric CO2 is complex and multifaceted. Ice sheets influence the solubility of CO2 in the oceans, acting as a carbon sink. As ice sheets melt, the solubility of CO2 in the oceans decreases, leading to an increase in atmospheric CO2. This, in turn, can further accelerate the melting of ice sheets.

Additionally, the melting of ice sheets can disrupt ocean circulation patterns, such as the Gulf Stream, which can have far-reaching consequences for global climate. The release of freshwater from melting ice sheets can slow down ocean currents, impacting heat distribution and affecting regional climates.

Overall, the interplay between ice sheets and atmospheric CO2 is a critical component of Earth's climate system, with far-reaching consequences for the planet's climate and ecosystems.

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