Why is action related to global warming so urgent? In this, our final section, we’ll be discussing the planet’s inertia, as well as climate feedbacks and tipping points.
To this point, we’ve noted that carbon dioxide is a primary driver of the Earth’s vital greenhouse effect and that human activities have pushed CO2 levels to their highest point in at least 650,000 years and at a rate that is likely unprecedented in the 4.5 billion year history of the planet.
We also noted that, despite considerable natural mechanisms working in opposition, both CO2 and temperature levels have continued to rise, and that these increased levels are reflected in multiple natural indicators around the world.
But why is action to reduce carbon emissions so urgent? The answer begins with discussing inertia.
A pot of water doesn’t boil immediately once heat is applied, nor does that water cool down immediately once the heat is removed. The same principle applies to the climate system. The impacts of a climate forcing are not always felt immediately but rather over time as the system reacts. As a result, even if all emissions halted immediately, the effects of current and previous emissions that have accumulated in the climate system would continue to be felt for decades.
“The feeling is that if things are getting bad, you hit the stop button. But even if you do, the climate continues to change.”
Dr. Gerald Meehl, U.S. National Center for Atmospheric Research (NCAR), March 2005
Think about the train shown in Figure 5-1. A small forcing continually applied slowly starts the train moving forward, gradually overcoming the forces of weight and friction working in opposition. Building momentum, the train slowly accelerates until it gets up to speed. Now imagine that the train has no brakes. Even if the accelerating force is removed, the train will continue on for a very considerable distance, propelled by its own momentum.
In addition to its inertia, the climate system contains feedback mechanisms. Feedbacks are natural responses to a condition that can either amplify that condition or diminish it. Amplifying feedbacks are called positive while diminishing ones are called negative.
Increased temperatures as a result of human carbon emissions can encounter both positive and negative feedbacks. As we discussed earlier, the ability of warmer air to hold more water vapor than colder air can enhance the greenhouse effect even further. It can also result in more clouds which can trap more heat near the Earth’s surface. As ice melts around the world, it also releases any gases trapped within that ice, including additional greenhouse gases. And, as this very reflective ice melts, it leaves behind underlying land masses and ocean waters that absorb much more heat than the lost ice
Working in opposition, the ocean is absorbing greater amounts of carbon dioxide at the cost of increasing oceanic acidity. Additional plant growth also removes greater amounts of CO2 from the atmosphere, providing an additional buffer for human emissions. And while increased cloud cover can trap additional heat, it can also increase the Earth’s reflectivity and promote cooling temperatures.
But some of these feedback mechanisms, both positive and negative, have limits, beyond which they can either kick in to overdrive or even switch from negative to positive.
These limits are called “tipping points”. Once one or more tipping points are crossed, self-sustaining changes on an immense scale become inevitable.
“We normally think of forests as putting the brakes on global warming, but in fact over the next few decades, damage induced by climate change could cause forests to release huge quantities of carbon and create a situation in which they do more to accelerate warming than to slow it down.”
Prof. Risto Seppälä, Finnish Forest Research Institute, April 2009
The absorption of CO2 by the world’s forests, the “lungs of the planet”. provides a massive cushion to emissions from human sources. But what happens if that uptake of carbon dioxide slows, or even reverses?
Enabled by warmer winters to flourish, bark beetles have infested and damaged or killed over 700,000 acres of forest in southern Wyoming. However, the epidemic may finally be slowing. The insects have simply run out of trees to attack.
As the trees die and decay, the carbon they contain is returned to the atmosphere as CO2. Since it began in 1996, the bark beetle infestation across Colorado and southern Wyoming has impacted 3.6 million acres in that region alone. In British Columbia, the epidemic has impacted over 35 million acres, a sample of which is seen in Figure 5-2, turning the trees from carbon sinks to carbon sources.
The equatorial rainforests are also susceptible. In 2005, as Hurricane Katrina made landfall in the U.S., the Amazon rainforest was enduring its worst drought in recorded history. A study found that half of the rainforest areas researched switched to emitting up to 12 times as much carbon per year as they were absorbing before the drought.
That historic 2005 event was then exceeded by an even larger drought in the Amazon in 2010.
In addition to the world’s forests, the earth’s oceans serve as a huge carbon sink. The colder the ocean water, the more carbon dioxide it can absorb from the atmosphere. So, the massive and frigid Southern Ocean surrounding Antarctica is a primary buffer for human carbon emissions.
But, like the forests, these oceanic sinks have limits. And, at least two studies, both published in 2007, found the sinks provided by both the Southern Ocean, seen in Figure 5-4, and the North Atlantic to be slowing.
Methane is twenty times more powerful in its greenhouse abilities than CO2, and around the world in the northern latitudes, huge stores of the gas are trapped inside of frozen permafrost. Methane is also very flammable, as shown in Figure 5-5, where a pocket of the gas in frozen ground is pierced and ignited.The National Snow and Ice Data Center defines permafrost, or permanently frozen ground, as soil, sediment, or rock that remains at or below 0°C for at least two years. Homes and roads are built on it. But, apparently, permanence is no longer a given as permafrost around the world is showing significant signs of melting. As the permafrost melts, it releases its stored methane back into the atmosphere.
And 55 million years ago, a huge change happened very, very fast in geologic terms in the global climate. Temperatures spiked by several degrees in just a few thousand years. The event is called the Paleocene-Eocene Thermal Maximum or PETM. A leading theory as to the cause of the PETM is that there was a massive release of methane from reserves on the ocean floor as ocean waters warmed. Considering the immense amounts of methane in natural reserves around the planet, the PETM provides a crucial example of the climatic impact of large-scale changes in atmospheric greenhouse gas concentrations.
“High latitude wetlands are currently only a small source of methane but for these emissions to increase by a third in just five years is very significant. It shows that even a relatively small amount of warming can cause a large increase in the amount of methane emissions.”
Dr. Paul Palmer, Edinburgh University, January 2010
So, why is action related to curbing our carbon emissions so urgent?
As they absorb greater amounts of human carbon emissions, the chemistry of the world’s oceans is being fundamentally altered, posing a massive threat to oceanic food chains and the human populations that rely on them. A glimpse into a possible future ocean is shown in Figure 5-6. Shown is a coral reef teeming with life off the coast of Castello Aragonese near Italy. However, only a few hundred yards away, CO2 from a volcanic vent increases the acidity of the water and renders a near life-less landscape.The Earth’s climate also does not stop on a dime. Changes driven by human actions have momentum in the climate system, and the full impact of our actions today will not be completely reflected in the global climate for years to come. This momentum can carry the climate system across multiple tipping points, beyond which natural feedback mechanisms will drive the global climate to extreme conditions while making any human actions toward mitigation largely irrelevant.
Lastly, all of human society is predicated on climate predictability. This consistency in the climate dictates where we live, how we grow our food, and where we get our water. As the climate changes, all aspects of our society that rely on the predictability of the climate will be drastically impacted. But it’s not just human civilization that is dependent on climate predictability, it’s animal and plant species as well. Some species are able to adapt or migrate based on environmental changes. But, as we’ve seen in the microcosm of oceanic dead zones, other species are not able to adapt fast enough and will die as a result. As species die, food chains that depend on them can collapse as well.
And we’re seeing signs of these changes already in increasing oceanic acidification, massive forest infestations, droughts and glacier loss that impact food and water availability, and the loss of species confined by their surroundings.