The Earth’s climate is changing at a rapid pace. The past five years are the warmest on record and the past decade is also the warmest on record (1). In fact, since the 1980’s, each successive decade has been warmer than any preceding one since 1880 (1).  

This warming trend is projected to continue with high confidence (1,2). The evidence of climate change is overwhelming, with the stability of the earths spheres and thus the future outlook of civilization severely threatened (1,2).

At Terra Spheres we feel more equipped to tackle challenges and face adversity when we understand the basics of the issue first hand, not what might be expressed or altered through additional sources.

With climate change being such a large, complex, and truly global issue, we have set out to provide a simplified overview of what climate is and how it is changing, by examining the main drivers of climate on our planet. The following article will lead you through a basic overview of the Earth’s climate, its main drivers, and historical changes.

While it would be impossible to cover all the complexities of the Earth’s climate in one article, we have attempted to breakdown the four primary components of Earth's climate system in a simplified, digestible manner while also providing links to sources for further explanation.

Weather or Climate?

Whereas weather refers to the condition of the atmosphere at any given time or place (primarily a function of temperature, precipitation, humidity, winds, and air pressure), climate is the long-term average (thirty years or more) of weather in a particular area. We will focus on global climate in this article. 

“Climate is what you expect, weather is what you get day-to-day” 

While we largely attribute climate and weather as being contained in the atmosphere, both are heavily influenced by innumerable and complex interactions between the Earth’s spheres. These spheres are: the lithosphere (rocks and solid components of the Earth), the biosphere (all living things, including you!), the hydrosphere (Earth’s water, of which frozen water can be differentiated as the cryosphere) and of course the atmosphere.

Throughout the article there will be references to observed data (e.g., land based measurements of temperature or satellite data) and proxy data. The latter refers to data derived from the sampling of ice cores, tree ring analysis, sediment analysis, and more, in order to determine how climate has changed on timescales of hundreds, to hundreds-of-thousands of years. 

What affects climate? Why does it change?

There are four primary determinants of the Earth’s climate: 1. Solar output 2. Distance from and orientation to the sun 3. Albedo and 4. The greenhouse effect. 

1. Solar output

The vast majority (over 99%) of the Earth’s energy budget comes from our sun. This energy is produced via nuclear fusion, with the sun emitting mainly shortwave electromagnetic radiation (predominantly ultraviolet, visible, and infrared radiation between 0.1-3 micrometers, as shown by Planck’s Law).

This shortwave radiation travels at the speed of light, reaching Earth in a little over 8 minutes where it is either scattered, reflected, or absorbed (depending on the wavelength of the radiation, the size and type of particles it interacts with, and the surface it interacts with) first by the atmosphere, and then the Earth itself.

For example, blue light (shorter wavelengths of visible light, see Figure 2 below) is scattered much more than longer wavelengths of visible light (red) in the atmosphere through a process known as Rayleigh scattering, producing Earth’s familiar blue skies.

Thankfully for life on Earth, the majority of the more harmful shortwave radiation is absorbed by the atmosphere (see atmospheric opacity in Figure 2). The shortwave radiation that does reach the Earth's surface and is absorbed, heats the surface it encounters. The surface then re-emits that energy as longwave (infrared) radiation. It is this infrared radiation which warms the lower levels of the atmosphere, driving weather and climate.

The energy emitted from the sun is fairly constant, with periods of greater and lower output, primarily influenced by sunspot activity over eleven year cycles. These cycles move from periods of increased sunspot activity (more solar irradiance) to decreased sunspot activity (less solar irradiance) and thus impact climate through fluctuations in the amount of received energy.

As shown in Figure 3, if variations in solar irradiance were the primary cause of observed global warming since the late 19th century, we would have seen a pause in global warming and a decrease in the average temperature of the Earth between 2000-2008.

2. The Earths orientation and distance to the Sun

The Earth’s orbit around, and orientation to, the sun changes on scales of tens-of-thousands to hundreds-of-thousands of years. These changes are known as the Milankovitch cycles, of which there are three. 1. The shape of the Earth’s orbit, known as eccentricity 2. The angle of the Earth’s axial tilt with respect to Earth’s orbital plane, called obliquity; and 3. The direction Earth’s axis of rotation is pointed, known as precession.

These orbital variations have been shown to drive glacial-interglacial cycles. However, the fact that the current cycle phase would indicate we should be in a global cooling period, and the extremely long time periods over which these cycles occur cannot explain the rapid rate of temperature rise over the past century and a half.

 3. Albedo

Albedo describes the reflectivity of a surface to solar irradiance, and in turn determines how much energy will be absorbed or reflected. The albedo of a surface is primarily determined by colour, surface roughness, and the angle of the sun towards the surface. 

When sunlight is reflected off of a surface (i.e. not absorbed), it does not affect the temperature of the surface. Surfaces with higher albedos will be cooler than those with lower albedos, and thus albedo plays a significant role in balancing the Earth’s energy budget.

The Earth’s albedo varies mainly as a function of cloud cover, snow, ice, foliage area, and land cover changes. Albedo is measured on a 0-1 scale, with an albedo of 0 meaning the surface absorbs all incoming energy (black) and 1 being a perfect reflector (white) reflecting all incoming energy. You have experienced the effect of albedo innumerable times, for instance the difference between wearing a black versus a white t-shirt in the summer. 

Changes in albedo, such as the reduction of areas covered in ice or changes in land cover can have large impacts on the amount of energy absorbed at the Earth’s surface, and thus is a primary influencer on climate. While it has acted as a positive feedback mechanism to increase warming, especially in the Arctic and other ice covered regions, changes in albedo are not sufficient to explain the onset of climate change.

4. The greenhouse gas effect

As mentioned earlier, energy from the sun can either be reflected, absorbed, or scattered by atmospheric gases. While there is strong scattering of the sun’s shortwave radiation in the atmosphere, the opposite is true of longwave radiation, with longwave radiation being more strongly absorbed in the atmosphere than shortwave radiation.

This is a result of selective absorptivity of different gases, in that different gases absorb some wavelengths very effectively (for instance in Figure 2 the opacity of the Earth’s atmosphere to UV is primarily a result of absorption due to ozone), some not at all (ozone is ‘transparent’ to visible light), and some partially. Gases that are most effective at absorbing shortwave radiation are oxygen, ozone, and water vapour. Those that are most effective at absorbing longwave radiation are water vapour, carbon dioxide, methane, and nitrous oxide (i.e. greenhouse gases!).

Shortwave radiation that passes through the atmosphere and is absorbed by the Earth’s surface heats it up. The Earth’s surface then re-emits this energy as longwave radiation, which is absorbed by greenhouse gases (GHGs)  due to their molecular structure, and then re-emitted to be absorbed by other atmospheric GHG molecules, radiated back into space, or absorbed again by the Earth’s surface. The net effect of these GHGs is to keep more energy within the Earth system, which would otherwise be radiated back into space. 

Without any GHGs in the atmosphere the average surface temperature of the Earth would be an inhospitable -18°C, instead of the approximately 15°C it is today, yielding a greenhouse gas effect of about 33°C (4).

As shown in Figure 4, the radiative forcing associated with GHGs has increased by 43% relative to 1990, and by 160% relative to the start of the industrial revolution. Figure 4 also shows that while CO₂ is not the most abundant GHG (water vapour), nor the most powerful in terms of an individual molecules ability to absorb and re-emit energy,  its long residence time (hundreds of years), high concentration in the atmosphere, and warming potential results in CO₂ being the most critical factor explaining the warming we have experienced thus far.

Additionally, as CO₂ concentrations increase and more heat is stored in the Earth climate system, more water is evaporated resulting in a greater GHG effect and more warming, producing another positive feedback loop.

While it is true that the natural carbon cycle is a much larger source of CO₂ entering the atmosphere than human activities, the CO₂ entering the atmosphere through the natural carbon cycle is balanced with CO₂ being removed through ocean uptake, rock weathering, land based photosynthesis, and more. The issue of climate change is not in the amount of CO₂ exchanged between the Earth’s spheres, but in the increase of the net concentration of CO₂ in the atmosphere, which in turn leads to more energy being stored in the Earth energy system.

Concluding thoughts

Hopefully this article helps those wanting to have a better understanding of the physical processes behind climate change and why increasing GHGs (especially CO₂) as a result of human activities has such dramatic effect on the Earth’s climate. 

We will be back soon with a continuation of our climate series, either focusing on the main drivers of weather and climate on the south coast of BC and what future climate change could look like in the region, what the Intergovernmental Panel on Climate Change is and how climate is studied, or what the future looks like based on our individual and collective decisions and how predictions have panned out thus far. 

Let us know which one would be of greatest interest to you!

We would love any feedback you might have, any topics you would like to learn more about or anything that needs to be clarified.

Get in touch and drop us an email at rrichardson@terraspheres.ca, or reach out to us on our Facebook and Linkedin pages.

Works Cited

1. World Meteorological Organization. (2020). WMO Statement on the State of the Global Climate in 2019. Geneva: WMO Publications Board.

2. Masson-Delmotte, V., Zhai, P., Pörtner, H. O., Roberts, D., Skea, J., Shukla, P. R., . . . Waterfield , T. (2020). IPCC, 2018: Summary for Policymakers. In: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global. Geneva: IPCC.

3. Kump, L. R., Kasting, J. F., & Crane, R. G. (2009). The Earth System (3rd edition). Upper Saddle River: Pearson.

4. Ross, S. L. (2013). Weather and Climate: An introduction. Don Mills: Oxford University Press.