The Earth is an intricate interconnected system of many physical, chemical and biological processes. Any external or internal change applied to the climate system, whether natural or manmade, can lead to additional change or have a knock-on effect to the initial perturbation, in a mechanism known as a feedback. Feedbacks further alter the climate, in some cases with greater impact than the initial forcing alone.

Natural changes have occurred many times in the Earth’s history, going back millions of years. A good example of this is shown by examining the growth and decline of the Earth’s ice sheets in line with the Earth’s orbital changes that take place over many thousands of years. These include changes in the Earth’s axis of rotation[1], the tilt of the Earth[2], and the shape of its orbit, known as eccentricity[3]. These orbital changes have resulted in increases or decreases in incoming solar radiation (known as solar insolation) leading to the decline and growth in planetary ice sheets in the northern and southern hemisphere, in well known cycles. Ice sheets grow when solar insolation is low. Ice is more reflective of the sun’s rays than darker surfaces such as land or the water, which tend to absorb more of the incoming rays. As the amount of ice covering the poles increases, more solar radiation is reflected back out to space in a process known as the ice-albedo feedback. This is turn causes more ice to form and more radiation reflected out to space.

The reverse of this process is also true. As ice sheets melt, more of the darker land and water surfaces are exposed leading to greater absorption of the incoming solar rays, further warming and greater ice melt. This is an important factor in today’s climate change where the rates of warming in the Arctic are higher than the overall average rate of warming of the globe. Whatever the initial cause of the atmospheric warming, a warmer planet leads to melting ice, which reliably melts at zero degrees Celsius. Note also that ice melt releases carbon dioxide and methane that would otherwise remain trapped beneath the ice, adding to the greenhouse gases in the atmosphere, which also have an additional warming effect.

There are other feedback mechanisms, which must be taken into account when predicting future climate change. A warmer atmosphere can hold more water vapor.

Since water vapor is itself a greenhouse gas it traps the surface heat and prevents it from escaping out to space. This leads to more atmospheric warming and further atmospheric water vapor, or humidity, and therefore greater and more intense precipitation in given regions (and corresponding decreased precipitation in other regions since there is always a fixed amount of water in the hydrological cycle). Note that the reverse situation is also true, in that a colder atmosphere holds less moisture, leading to further cooling and further decreases in atmospheric humidity or moisture. However, of all the greenhouse gases in the atmosphere, although it is the most abundant, water vapor stays in the atmosphere only for a short time and eventually condenses out as rain or snow. Other powerful greenhouse gases such as carbon dioxide and methane stay in the atmosphere for decades or even centuries, contributing greatly to long term warming. Any increase (or decrease) of water vapor in the atmosphere is a feedback, since it follows the initial temperature increase (or decrease) and amplifies (or dampens) the initial effect.

In a warming world, the ice-albedo, water vapor and other so-called amplifying feedbacks increase the effects of the initial warming. There are also dampening feedbacks that offset the initial warming. One example would include the so-called lapse-rate feedback.

The lapse rate is the rate at which temperature decreases as one ascends upwards in the troposphere, away from the surface. Warmer air higher up in the troposphere can radiate heat away to space more easily than warmer air near the ground, since air closer to the ground has to travel further through the atmosphere before it reaches the top of the troposphere. As the planet’s temperature warms, the temperature is predicted to increase throughout the troposphere, with greater relative increases higher up in the troposphere than near the surface. According to thermodynamic laws, a hotter body radiates greater heat than a cooler body, so as the temperature of the troposphere increases, its ability to radiate the heat also increases.

Another dampening feedback results from the introduction of microscopic particles (known as aerosols) into the atmosphere. Natural volcanic eruptions release sulfur particles high into the atmosphere where they act like miniature mirrors and reflect the incoming solar rays before they reach the surface the earth, leading to surface cooling which can last for several years. Eventually these particles fall to Earth and the cooling effect ceases. Man-made atmospheric particles include carbon, soot, ammonia, sulfate and nitrate compounds. Each particle type has varying and complex effects on the climate, depending on its location in the atmosphere. However, note that although atmospheric particles generally have a cooling effect, they also lead to changes in cloud properties, since aerosols act as the initial ‘seed’ with which water condenses and forms clouds.

Many factors determine the direction of the climate’s feedback response to clouds (whether amplifying or dampening), including cloud type, cloud height, cloud surface characteristics, season and location. The effect of clouds in a warming world is particularly complicated since clouds cause both warming and cooling. For example, thick low-level clouds have both a cooling effect by reflecting sunlight back into space but also a warming effect by trapping some radiation and radiating it back to the Earth’s surface. However, high-level cirrus clouds allow the sun’s rays to reach the Earth’s surface, but provide little cooling so have an overall effect warming effect.

The majority of the studies suggest that long-term cloud feedback has an amplifying effect resulting in increased warming.

Finally, the earth’s carbon cycle which can be defined as the natural circulation of carbon (in its many forms including carbon dioxide) among the atmosphere, oceans, soil, plants, and animals is also affected by a warmer (or indeed colder) atmosphere. There is evidence to show that a warmer atmosphere leads to an increase in the amount of atmospheric carbon dioxide. This produces further warming, and so the carbon cycle feedback is an amplifying one.

Over the course of its history the Earth has experienced many glacial periods or ice ages (with the existence of large ice-sheets) and interglacial periods, due to orbital changes around the Sun. The Earth is currently in an interglacial period. Paleoclimatic evidence shows that warming at the end of glacial periods was more abrupt than would be expected due to the increase in solar insolation alone. There are several positive feedbacks that are responsible for this, including both ice-albedo and carbon cycle feedbacks. Direct measurement of past carbon dioxide trapped in ice core bubbles (with clear identification of the source of the carbon dioxide from the presence of specific carbon isotopes) show that warming at the end of the glacial periods released carbon dioxide from the ocean, which strengthened the atmosphere's greenhouse effect and contributed to further warming.

All the feedback mechanisms described above create an effect on the climate system, which is additional to the initial external change (known as a forcing). Climate change feedbacks add to the challenge of quantifying future temperature change predictions, and themselves must be taken into account when using any climate modeling techniques. Some of the feedbacks, such as water-vapor, ice-albedo (in the case of sea-ice) and lapse-rate are considered fast feedbacks, since they respond rapidly to changes in surface temperature. Their effect on the overall Earth’s energy budget is almost instantaneous. Slower feedbacks that take decades or longer to impact the energy budget (such as ice-albedo in the case of ice sheets) are equally important. However, their effect is harder to quantify given the limited amount of data available with which to observe them.

Water vapor and ice-albedo feedback are fairly well understood by climate scientists and their effects are routinely included in climate models. Feedbacks such as the carbon cycle and cloud feedbacks are less well understood, so are included in models with a larger degree of uncertainty. Further feedback mechanisms, such as methane hydrates (methane reserves locked away in the depths of the oceans) and permafrost methane, are not yet well understood and not widely used in current climate models. Methane is a greenhouse gas more than 20 times as potent as carbon dioxide, so any methane releases could have serious implications for the climate.

Scientists also assume that there are hidden feedbacks that are as yet unknown. As Earth’s climate moves further away from the norm, we are exposing ourselves to more risk from these unknowns.                             

 

 

[1] This phenomenon is known as ‘precession’ or ‘precession of the equinoxes’ and can be likened to a spinning top that wobbles around its axis of rotation as it is spinning. The Earth’s axis of rotation performs a full circle every 25,800 years.

 

[2] Currently tilted at an angle of 23.5 degrees but varies on a 41,000 year cycle between 22.1 and 24.5 degrees. Also known as the ‘obliquity.’ It’s this tilt that gives the Earth its seasons in the northern and southern hemispheres.

 

[3] The ‘eccentricity’ of the Earth’s orbit determines how circular or elliptical the Earth’s orbit is around the Sun. The other planets in the solar system exert their influence on the Earth and affect its eccentricity.  A more elliptical orbit affects the length of seasons on Earth. A perfectly circular orbit would result in the seasons all with the same duration.