Chapter 2: Natural Climate Change: Volcanoes and the Sun
So, we’ve established that orbital parameters, like eccentricity of Earth’s orbit and the magnitude of the tilt of the planet are not responsible for the current and rapid climate change that we are seeing. What about the Sun, the source of energy for the Earth’s climate system? Although the Sun’s energy output appears constant from an everyday point of view, small changes over an extended period of time can lead to climate changes. Scientific studies demonstrate that solar variations have played a role in past climate changes. For instance, a decrease in solar activity was thought to have triggered the Little Ice Age (a cool period in the Northern Hemisphere) between 1650 and 1850, when Greenland was largely cut off by ice from 1410 to the 1720s and glaciers advanced in the Alps.
What about current global warming? The Sun’s energy fluctuates on a cycle that’s 11 years long – and the energy changes only by about 0.1% during each cycle. Since 1750, the average amount of energy coming from the Sun either remained constant or increased slightly. Since 1870, solar fluctuations have contributed a maximum of only 0.1 degree Celsius to temperature changes. Scientific observations of solar activity, in fact, have shown a slight cooling trend since 1960, with the last few decades showing the sun/climate trend moving in opposite directions. It is clear that the warming we are seeing on the Earth is not due to solar activity. Additionally, if global warming was caused by a more active sun, scientists would expect to see warmer temperatures in all layers of the atmosphere. Yet, they have observed cooling in the upper atmosphere, and warming at the surface and in the lower parts of the atmosphere, due to greenhouse gases like carbon dioxide, methane and water vapor. Greenhouse gases are so named because they can capture heat. If you want to feel this for yourself, walk into a greenhouse on a hot day! Climate models that include solar irradiance changes cannot reproduce last century’s observed temperature trend without including a rise in greenhouse gases.
Then what about volcanoes, known for spewing greenhouse gases in the form of carbon dioxide? In the grand scheme of things, volcanoes actually have a short-term cooling effect (about 0.1 – 0.2 degrees C) on the climate, as they pump out dust and ash which can temporarily block out sunlight. Volcanoes also spew out sulfur dioxide gas which, when combined with water vapor and dust in the atmosphere, forms sulfate. Sulfate aerosols actually reflect sunlight AWAY from the Earth’s surface, and so their cooling effect outweighs warming caused by emitted volcanic greenhouse gases. In 1991, when Mount Pinatubo erupted in the Philippines, it caused a 0.5 C drop in global temperature! This cataclysmic eruption was the second-largest volcanic eruption of this century. Although, yes, volcanoes do emit carbon dioxide, a prominent greenhouse gas in our atmosphere, their average emissions are less than a 1% contribution than that from current human emissions. The status of the Sun and the outputs from volcanoes are not responsible for our current rising temperatures.
The Earth revolves around the Sun in an overall elliptical orbit. No, this isn’t the same thing as the elliptical you find in your gym (because if you Google “elliptical”, those are the first results that show up!) Rather, it’s the shape of an orbit and, in the case of the Earth, the shape can vary through time – sometimes becoming more circular and sometimes becoming more like an oval. The technical term for this is eccentricity and it changes how much sunlight we can receive on planet Earth because there are times of year when the planet is closer to the Sun than at other times. Quite simply, when Earth is closer to the sun, it receives more solar radiation. There are two periodicities that have been identified: one cycle is an average of 100,000 years and another, longer, cycle is about 413,000 years.
The Earth itself is slightly tilted, too (this is obliquity), and the amount of tilt changes with time. More tilt means warmer summers and colder winters; less tilt means cooler summers and milder winters. The periodicity of this change is on the order of 41,000 years. The direction of tilt (precession) also changes – towards or away from the Sun, on a 19,000 – 23,000 year timescale. These shifts and wobbles in the Earth’s orbit can trigger changes in climate, such as the beginning and end of ice ages. But orbital changes are so gradual that they’re only noticeable over thousands of years – not decades or centuries.
The slow changes in the Earth’s orbit lead to small but climatically important changes in the strength of the seasons over tens of thousands of years. Any other climate feedbacks in the system can amplify these small changes. In the shift between glacial and interglacial periods on Earth, this is most related to the severity of summers in the Northern Hemisphere (which has more land than water and land warms faster than water). When summers are mild, enough snow and ice remain throughout the season, maintaining glaciers. When summers are too hot, more ice melts in the summer than can be replenished in the winter. A “perfect orbital storm” for global warming would require Earth’s orbit at its highest eccentricity, Earth’s axial tilt at its highest, and the Northern Hemisphere in “perihelion” (its closest point) at summer solstice. Rather, in our current configuration, the Earth’s Northern Hemisphere currently experiences its summer in aphelion (its farthest point from the Sun), the planet’s tilt is currently on the lower end, and the Earth’s orbit is fairly circular. Earth’s current orbital positions within these cycles, thus, should result in cooler temperatures, but instead, the average temperature of the planet is on the rise. What we’re seeing isn’t natural climate change due to orbital parameters.
To be continued…