Introduction
On occasion, a national news outlet will run a headline covering global warming, and your local station will cover recent temperature anomalies. One that has popped up on my phone multiple times and in different varieties: "Scientists say we have until [insert date] to decrease temperatures by [insert degree] or else [insert catastrophic event]". Cryptic declarations like that can bring a hopeless and sunken feeling – for good reason. The implications of such a worldwide transition will undoubtedly change just about everything we take for granted. This exisentital burden has framed humans responsible for the survival of all other life on Earth, as either their saviors or destroyers. Much like the fear of nuclear weapons and unpredictable earthquakes, the fear of global warming eventually leads to a subtle realization and acceptance: it's largely out of our individual control. And after reading a headline like that too many times, to the point of desensitization, it can leave us with more questions than answers. Often left out of these news reports is the Earth's climate history of the past 10,000, let alone 1,000 years. Looking back is essential for understanding the situation we're in now, and perhaps more importantly, the future that lies ahead.
Arguments
The peak of the last glacial period occurred between 25,000 and 18,000 years ago. The warming of that time, which took thousands of years, was due to the Milankovitch Cycle. The ice sheets in the northern and southern latitudes melted and retreated to the Earth's poles, seen today in the Antarctica and Greenland. Between the Last Glacial Maximum (LGM) and our current geological epoch, the Holocene, the sea level has risen due to the melting of ice. Jumping forward to the 18th century, the Industrial Revolution kicks off. Most machinery require a fossil fuel (coal, natural gas, and petroleum) as a power source. When burned, they emit greenhouse gases like carbon dioxide (CO2), which creates a strong insulation across the Earth's atmosphere. As such, this greenhouse effect traps heat in the Earth's surface and increases the average global temperature.
Evidence
The Milankovitch Cycle
Around 18,000 years ago, the LGM gradually came to a close when the high latitudes of the Northern Hemisphere experienced an increase in solar insolation during the summer seasons. This was due to the Milankovitch Cycle, which explains how glacial and non-glacial cycles arise by natural variations in the eccentricity, precession, and obliquity (axial tilt) of the Earth. With a cycle length of 100,000 years, the orbit of the Earth becomes more eccentric and allows entry for varying amounts of solar radiation. When the orbit is circular, the amount of solar radiation is constant. As the orbit becomes elliptical, a season on Earth will change as the distance between the Sun and the Earth increases or decreases. The amount of solar radiation will not remain constant throughout the year, in turn allowing temperature variation on Earth. Every 23,000 years, Earth’s axis will complete a full precession cycle, a shift in the orientation of an axis of rotation. The precession of equinoxes is why the aphelion, which is when the Earth is farthest from the sun, has been during the Northern Hemisphere summer; when the cycle restarts, the aphelion may take place during the Northern Hemisphere winter. When the Earth’s obliquity is not as large, the difference in temperature between summer and winter is practically indiscernible. The angle swings between 22.1° and 24.5° on a 41,000 year cycle, and must be 24.5° in order to make a large difference.
The Younger Dryas period
The transition between the LGM and the Holocene was a period of melting ice sheets and increasing sea levels. The length of summers had increased after the LGM. During the Pleistocene, the four major ice sheets—Laurentide, Cordilleran, Greenland and Fennoscandian—covered a large portion of the Northern Hemisphere. Inadequate snowfall caused these ice sheets to melt, which raised the the sea level about 100 meters. Almost 5,000 years after the end of the LGM, the warming conditions had reversed back to cooling; this is known as the Younger Dryas (YD) period. Taking place 12,900 to 11,700 years ago, the YD was not caused by orbital variations, but rather the melt of cold freshwater releasing in the North Atlantic. These conditions were most optimal for ice formation, due to freshwater freezing faster than saltwater. The initial evidence of YD was the resurgence of Dryas, an arctic evergreen plant that only grows in cold temperatures. As the cooling conditions ceased, the natural transition to the Holocene was quick. Ice core records show that the Earth warmed 7°C / 44.6°F only 50 years after the Younger Dryas.
The Greenhouse Effect
Anthropogenic climate change is caused by the recent surge of greenhouse gases, which are released into the atmosphere when fossil fuels are burned. First, it is important to understand the fundamental processes of the greenhouse effect. The gases—Carbon Dioxide (CO2), Methane (CH4), and Nitrous Oxide (N2O)—are naturally occuring compounds and the byproducts of combustible fuel. Humans and plants take in oxygen (O) and release CO2. It is incredibly important for the Earth to have atmospheric CO2. Unlike other compounds or elements found in Earth's atmosphere, greenhouse gases are not transparent to any long wave heat energy; they are absorbed and reradiated. In layman's terms, when heat is released from Earth's surface, the energy floats upward through the atmosphere; the three gases ping the heat back to the surface, keeping the Earth warm and suitable for life to flourish. However, as humanity burns fossil fuels, greenhouse gases are emitted at a much higher rate than most natural process. This has caused the sudden increase of average global temperatures. Human civilization relies heavily on fossil fuels. A typical year will see 25% of fossil fuels being used in electricity and heat production, 24% in agriculture, 21% in industry, 14% in transportation, 10% in other energy and 6% in buildings. Contemporary evidence of anthropogenic climate change was revealed over 70 years ago by scientist Charles Kealing, who found a way to measure the amount of greenhouse gases in the atmosphere. Another indicator of rising atmospheric CO2 levels are found in ice cores, which can date hundreds of thousands of years into the past. Scientists examine the air bubbles trapped within the ice. In Vostok, Antarctica, scientists discovered that every 100 years, atmospheric CO2 levels swing from 300 parts per million per volume (ppmv) down to 180 ppmv. In the present-day, that number is 417 ppmv.
The large increase in atmospheric greenhouse gases and the subsequent global warming has brought about a number of discernable changes to the planet. The primary effect is the increase in global temperatures: from 1900 to the present, average temperatures increased between 2° C to 4° C. Scientists estimate that by 2100, average temperatures will increase by 0.5° C. The high latitudes are warming the fastest, dubbed by climate researchers "Arctic Amplification". Snow cover in the Northern Hemisphere is expected to decline heavily; the melting glaciers and ice caps will raise the sea level. At the current rate, by 2100 the sea level in Southern California will have risen by one meter.
Conclusion
- The Last Glacial Maximum ended due to natural variability in the Milankovitch Cycle.
- The warming between the LGM and the Holocene took thousands of years.
- Large ice sheets melted and the sea level rose about 100 meters.
- Anthropogenic climate change is due to the burning of fossil fuels, which emits greenhouse gases that trap heat in the Earth's atmosphere.
- Global temperatures have risen by 2°C since 1900.
- Global temperatures are expected to increase by 0.5°C in 2100.
- CO2 concentration in the atmosphere affects the high latitudes.
- Glaciers and ice caps are expected to melt in the next hundred years.
- Sea level rise will be greater in some parts of the world than others.