“You Asked” is a series where Earth Institute experts tackle reader questions on science and sustainability. In honor of Climate Week NYC and the Covering Climate Now initiative, we’ll spend the next few weeks focusing on your questions about climate change.
The following question was submitted through our Instagram page by one of our followers. The answer was provided by Bärbel Hönisch.
Prior to the Industrial Revolution, what caused climate change on earth?
CO2 is a gas that is used and produced by many processes, including photosynthesis; consumption of organic matter by bacteria, fungi and animals (i.e. respiration); production and dissolution of shelled marine organisms, volcanism, weathering of rocks on land, burning of fossil fuels, and concrete production. These processes operate on different time scales, and the relative dominance of CO2 sources or CO2 sinks determines whether atmospheric CO2 concentrations increase or decrease over time.
Photosynthesis and respiration processes globally tend to balance each other, such that CO2 sequestered by photosynthesis is returned to the atmosphere by respiration. This can be seen in the in the seasonal wiggles recorded at the Mauna Loa Observatory on Hawaii; the wiggles reflect the seasonal greening of boreal forests in Earth’s northern and southern hemispheres.
On geologic time scales (i.e. hundreds of thousands to millions of years), plate tectonics and the repositioning of continents cause volcanism, mountain-building and large-scale climate changes such as glaciation, humid periods (i.e. green periods with intense photosynthesis) and desertification. These processes can create environmental conditions that favor one process over the other. For instance, during the Carboniferous period (~340 million years ago), large expanses of wetlands protected dead plant material from microbial decay, thus trapping and storing carbon. Consequently, photosynthesis dominated over respiration, atmospheric CO2 concentrations dropped to a minimum, and large amounts of plant carbon stored in those swamps ultimately formed the coal beds that we are burning today to produce much of our electricity. This formation of coal, oil and gas deposits takes tens to hundreds of millions of years.
Similarly, volcanism releases CO2, whereas weathering of rocks on land consumes CO2, and depending on which process dominates, atmospheric CO2 concentrations either increase or decrease. For example, massive volcanism at the end of the Permian period (~250 million years ago) sent atmospheric CO2 soaring. In contrast, atmospheric CO2 decreased after India collided with Asia some 40 million years ago. Scientists hypothesize that weathering of exposed fresh Himalayan rocks has pulled CO2 out of the air, resulting in the relatively low concentration of CO2 in our atmosphere over the last 15 million years. Finally, cyclic changes in the orientation and shape of Earth’s orbit around the Sun every 20,000 to 100,000 years also affect climate, by changing ocean circulation, algae productivity and thereby atmospheric CO2. These changes are responsible for the cyclic CO2 variations we observe in ice core records of the past 800,000 years.
After a perturbation of the natural carbon cycle, such as massive volcanism or the anthropogenic burning of fossils fuels, several processes set in to neutralize excess atmospheric CO2. The ocean is the first to respond. It absorbs elevated atmospheric CO2 on the time scale of decades to hundreds of years, and seawater thereby acidifies. This acidification causes deep ocean CaCO3 deposits (i.e. fossil shells) to dissolve, a process that takes hundreds to thousands of years and also consumes CO2. Weathering of rocks on land ultimately restores the low CO2 concentrations that existed before the perturbation, but this process takes much longer — tens of thousands of years. These are very long time scales when one considers how much societies have evolved over just the past 1,000 years.
Currently, anthropogenic fossil fuel burning releases CO2 from millions-of-years-old geologic reservoirs within a few decades and centuries, and the CO2 rise is further aggravated by deforestation, agricultural practices and concrete production. It has been estimated that humans are releasing CO2 ten times faster than the largest geologic carbon cycle perturbation of the past 60 million years, and that neutralizing the excess anthropogenic CO2 via weathering of rocks on land will take hundreds of thousands of years. At that rate, the CO2 emissions that humans have produced since the onset of industrialization in the 1850s, and which will continue for at least another century or two, will affect 12,000 generations of people. Consequently, and from the human standpoint, anthropogenic CO2 emissions will change Earth’s climate ‘forever’ if we rely on natural CO2 sequestration processes only. Drastically reducing emissions and deploying engineering methods to efficiently remove CO2 from the atmosphere can change this prospect, but the effort and cost required to do so increase the longer we wait.
This response was adapted from an FAQ that Hönisch wrote for paleo-CO2.org.