Without the greenhouse effect, Earth's average temperature would be about $-18\ \text{°C}$: a frozen planet. So why do we talk about the greenhouse effect as a problem? Because we are intensifying it much faster than ecosystems can keep up with.
The natural greenhouse effect
The Moon is the same distance from the Sun as Earth, but it has no atmosphere: its average temperature is about $-20\ \text{°C}$, while our planet's is around $15\ \text{°C}$. What mechanism in the atmosphere holds in heat and keeps the surface from being a frozen desert?
The main GHGs are water vapor, carbon dioxide ($\text{CO}_2$), methane ($\text{CH}_4$), and nitrous oxide ($\text{N}_2\text{O}$). Each one has different sources:
Gas
Main source
$\text{CO}_2$
Burning fossil fuels, deforestation, fires
$\text{CH}_4$
Livestock (ruminants), rice paddies and wetlands, landfills, gas leaks
$\text{N}_2\text{O}$
Nitrogen fertilizers in agriculture, industrial processes
Methane and nitrous oxide trap much more heat per molecule than $\text{CO}_2$, but $\text{CO}_2$ contributes the most to current warming because it is emitted in enormous quantities and stays in the atmosphere for centuries.
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Observe the greenhouse effect diagram. Explain what each numbered arrow (1, 2, and 3) represents, why the natural greenhouse effect is essential for life on Earth, and…
Observe the greenhouse effect diagram. Explain what each numbered arrow (1, 2, and 3) represents, why the natural greenhouse effect is essential for life on Earth, and what human-induced alteration consists of.
Remember the carbon cycle: photosynthesis removes $\text{CO}_2$ from the atmosphere, and respiration, decomposition, and combustion return it. For millennia that exchange was almost in balance. Since the Industrial Revolution, humans have been taking carbon from a reservoir that was isolated (coal, oil, and gas) and moving it into the atmosphere, and we have also reduced the forests that absorb it. The two human processes that contribute the most are burning fossil fuels and deforestation (along with land-use changes).
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Relate the carbon cycle to the increase in atmospheric $\text{CO}2$ since the Industrial Revolution, naming the two human processes that contribute most to this increase.
Relate the carbon cycle to the increase in atmospheric $\text{CO}_2$ since the Industrial Revolution, naming the two human processes that contribute most to this increase.
The concentration of atmospheric $\text{CO}2$ has gone from around $280\ \text{ppm}$ in pre-industrial times to around $420\ \text{ppm}$ today. Calculate the percentage…
The concentration of atmospheric $\text{CO}_2$ has gone from around $280\ \text{ppm}$ in pre-industrial times to around $420\ \text{ppm}$ today. Calculate the percentage increase and discuss what would have happened to ecosystems if this increase had occurred over centuries instead of less than two.
In a debate on climate change, someone asserts: "The climate has always changed due to natural causes. Moreover, because warm water holds less dissolved gas, the…
In a debate on climate change, someone asserts: "The climate has always changed due to natural causes. Moreover, because warm water holds less dissolved gas, the contemporary increase in atmospheric $\text{CO}_2$ is a consequence of natural ocean warming and not of human activities."\na) Compare the rate of current warming with that of climatic transitions in the geological past (such as Quaternary deglaciations) and deduce why the speed of modern change rules out Milankovitch orbital cycles as the primary cause.\nb) Use the equilibrium of the marine carbonate system and ocean acidification data (continuous decrease in seawater pH) to refute the idea that $\text{CO}_2$ originates from oceanic outgassing.\nc) Explain how the observation of tropospheric warming coupled with stratospheric cooling makes it possible to rule out an increase in solar activity as the cause of the change and confirms the role of the greenhouse effect.
Imagine that global warming exceeds the $+1.5\ \text{°C}$ threshold, triggering accelerated processes in high northern latitudes.\na) Model the interaction between the…
Imagine that global warming exceeds the $+1.5\ \text{°C}$ threshold, triggering accelerated processes in high northern latitudes.\na) Model the interaction between the summer melting of Arctic sea ice and the oceanic radiative balance using a cause-and-effect flow chart. Predict the impact of this loop on heat absorption by the water and on ice formation in subsequent winters.\nb) Thermokarst lakes form in peatlands and thawing permafrost soils. Reason under what biogeochemical conditions the decomposition of organic matter will predominantly release $\text{CH}_4$ instead of $\text{CO}_2$, and justify, based on the relative radiative forcing capacity of both gases, why this worsens short-term forcing.\nc) Based on both mechanisms, justify what a "tipping point" is and argue why, once crossed, the total cessation of anthropogenic emissions might not be sufficient to stop warming.
A city can bet on fleets of electric trucks to reduce emissions, or on planting shade trees and building drainage against floods. Why is it necessary to distinguish between acting on the source of warming and preparing for its unavoidable consequences?
Measures are applied at different scales:
Strategy
Example
Scale
Mitigation
Solar panels and wind farms (Oaxaca, Yucatán)
National
Mitigation
Reforestation and forest management in a community
Local
Mitigation
International agreements such as the Paris Agreement
Global
Adaptation
Restoring mangroves as a barrier against hurricanes
Local / regional
Adaptation
Rainwater harvesting and drought-resistant crops
Local
Adaptation
Early warning systems for hurricanes (SMN)
National
A single action can bring co-benefits: a Metrobús or a bike lane reduces emissions and also improves the city's air and people's health.
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Differentiate between mitigation and adaptation strategies for climate change and give two specific examples of each, indicating at what scale (local, national, or…
Differentiate between mitigation and adaptation strategies for climate change and give two specific examples of each, indicating at what scale (local, national, or global) they are applied.
Design a mitigation plan for your local area: choose three sectors (for example energy, transport, and carbon sinks) and justify, for each proposed action, how it…
Design a mitigation plan for your local area: choose three sectors (for example energy, transport, and carbon sinks) and justify, for each proposed action, how it reduces emissions or increases removals and what co-benefits it brings.
Back to the beginning: the atmosphere traps heat thanks to greenhouse gases; that natural effect keeps us at about $15\ \text{°C}$, and what changed is that we are adding more gases than ecosystems can absorb.
Let's practice! 3 medium exercises in this section