By Libby Murphy
Variability is a natural part of Earth’s intricate climate system. The planet has endured periods of warmth and glaciation alike in its some 4.5 billion year history. Shifts in climate can be traced back to a variety of natural events and trends, Earth’s rotation, solar irradiance, volcanic activity and more. Currently we are in a period of warming, the rapidity of which is unprecedented, unlike anything we have witnessed or have evidence of in the past. This global warming trend is abundantly clear, measurable and rarely disputed. What is debated, however, are the cause and affect of this warming. There is a strong consensus among the scientific community that this rapid warming is human induced. More specifically, that it is a result of anthropogenic emissions of greenhouse gases. To many people, however, this conclusion is not as evident or logical. The argument to support global warming ranges from a couple of fundamental concepts, the greenhouse effect and the carbon cycle, to key evidence connecting human activity to warming temperatures. An understanding of these issues is necessary to wrap one’s mind around the current global warming scenario. This paper intends to present these arguments in a manner more approachable to a non-specialist and will conclude with a framework for which to look toward the future and make key decisions regarding this issue, if you so desire.
Many fuels, including oil, wood, biomass, coal are made up almost entirely of carbon. When we burn these fuels, carbon reacts with oxygen to create carbon dioxide. This carbon dioxide is in gas form so it rises and settles in the atmosphere. The physical tendency of atmospheric carbon dioxide is to absorb and reradiate energy, effectively creating an insolating blanket around the planet. This in turn leads to warming on the Earth’s surface known as the greenhouse effect. This is the cause of global warming and can be measured in the steady rise of global mean temperature. Other greenhouse gases include methane, CFCs, water vapor and more. Currently, there is a large amount of greenhouse gases being emitted into the atmosphere as a result of human industrial activity, most significantly carbon dioxide from fossil fuel combustion.
Now that we better understand the cause and effect of global warming, one might wonder why it is so unusual for such an amount of carbon to exist in the atmosphere. Why is it such a big deal if humans reshuffle the carbon in our Earth system? Well, our earth has an excellent system for circulating carbon that is known as the carbon cycle. The cycle is made up of two major systems: the short-term surface cycle and the long-term rock cycle. The latter holds 99.9% of the carbon on Earth. The surface cycle comprises all of the carbon in the ocean, atmosphere, soil and biomass. Although the surface reservoirs only hold a small fraction of total carbon, it plays a much greater role in human-scale climate processes. The surface reservoirs cycle carbon through various processes including photosynthesis, respiration, dissolution and more. Carbon regularly flows between surface and rock reservoirs over long time periods of hundreds of thousand to millions of years through the slow processes of weathering, calcification, burial, metamorphism and more. Such a slow and significant reservoir, the rock carbon cycle is considered to act as a thermostat for Earth. This self-regulating likely played a central role in the ability of life to form on this planet in the first place. Without this balance, Earth may have experienced a runaway greenhouse effect making life an impossibility. The human act of mining and combusting fossil fuels removes carbon from the long-term rock reservoir and puts it directly into the short-term carbon cycle. This injection of carbon dioxide into our atmosphere is rapid and one-way, with no sufficient means of return. Such an abrupt disturbance is the driving force behind the global warming we previously discussed.
The steady and synchronized increase in both temperature and atmospheric carbon dioxide was first plainly observed by Charles David Keeling at the Mauna Loa Observatory in Hawaii. These observations began in 1958 and create what is famously known as the Keeling Curve.Scientists have since extrapolated these observations thousands of years into the past using ice cores, which present an incredible source of insight to our past climate because of the presence of air bubbles. These bubbles have effectively preserved the composition of our atmosphere going back in chronological order as you go deeper into the core. It is thanks to this that we now know that carbon dioxide is more prevalent in the atmosphere today than the last 800,000 years and is increasing at a rate 100 times that of ever before. It is hard to imagine such a trend could be due 100% to natural variation, especially considering that warming is occurring more on a human time-scale than geologic. Furthermore, the acceleration of this warming coincides impeccably with the onset of the industrial revolution.
In addition to our measurements of temperature and carbon dioxide, we can model the effect of greenhouse gases compared to other external factors that do work on the climate system, known as forcing factors. We measure the changing impact of each forcing factor and apply the numbers to a trend known as a regression model. Unlike climate models, which are useful but more speculative, these trends allow us to actually attribute global warming to these factors based on hard data. A recent regression by Lean nd Rind (2008) looked at various natural and anthropogenic forcings between 1889 and 2006. The results were staggering, only 16% of the temperature trends could be attributed to natural forcings with the bulk, 84%, being anthropogenic. The astonishing part about these findings is that they are based on actually measurements, not climate models. This along with other real-world observations, like the Keeling curve, is why there is such a strong scientific confidence behind anthropogenic global warming.
So even after you agree that global warming exists and is human-induced, what does that actually imply for our life here on Earth? Well, it simply means that we should expect changes in our climate, both long- and short-term. Climate change, unlike global warming, is far more difficult to measure because it relies on so many complex interactions and feedbacks in the system. Any predictions you hear about climate change are just predictions as no true climate scientists could possibly guarantee the outcome of global warming. We can do our best to estimate and model possible scenarios but we will never know for sure until we live it. However, we are already witnessing shifts that are consistent with our projections, like increased intensity and frequency of storms, droughts and floods. The important thing to remember is that just because we cannot know what climate change will bring, we do know that it is a threat. Humans are naturally risk-adverse; we take out insurance on almost every risky aspect of our lives, even if the odds are not that bad. It would be contradictory to not take out an insurance plan on climate change, the risk that threatens the entire globe. The effects of climate change are uncertain but how much greenhouse gases we emit is in our hands. As with any other major risk, we should consider the serious but uncertain threat of climate change and protect ourselves by reducing the odds against us. It will be difficult but we can achieve this by minimizing our impact and curbing our emissions. It is time to take this conversation from one of science to one of risk.