Climate change

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.

Name three greenhouse gases, state the main source of each, and indicate which contributes most to current warming and why.

Distinguish between weather and climate, and explain why a week of intense cold in winter does not contradict the global warming trend, drawing on the difference between an isolated data point and a long-term average.

Name two pieces of evidence of climate change observed in Spain (the retreat of the Pyrenean glaciers, earlier flowering, more frequent heat waves, rising sea level on the coast) and explain what each one consists of.

Describe two consequences of climate change already observable in the ecosystems of the Iberian Peninsula and explain the physical or biological mechanism that produces them.

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 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 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.

Explain what the albedo of a surface is, which surfaces on Earth have a high albedo and which have a low one, and why the melting of Arctic sea ice reduces the planet's average albedo.

Sea level rises for two main reasons: the thermal expansion of water as it warms and the input of water from glaciers and continental ice sheets. Explain both mechanisms and discuss, using Archimedes' principle, why the melting of floating sea ice barely contributes to the rise.

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.

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.

Many countries have committed to "carbon neutrality" by 2050. Explain what reaching net zero emissions means, what role removals by sinks play in the balance, and why it does not mean stopping emitting altogether.

To have a $50\ \%$ probability of limiting warming to $1{,}5\ \text{°C}$, the remaining global carbon budget is estimated at around $250\ \text{Gt}$ of $\text{CO}_2$. Currently, humanity emits approximately $40\ \text{Gt}$ annually.\na) Prove mathematically that, if global emissions were reduced linearly starting today until reaching net zero, the time available to reach neutrality without exhausting the budget of $250\ \text{Gt}$ would be only $12{,}5$ years (compared to $6{,}25$ years if they remained constant). Deduce the implication of this time lag for immediate mitigation policies.\nb) Explain why cumulative warming depends on cumulative historical emissions and not on the annual emission flow, deducing why stabilizing emissions at a constant value does not halt temperature rise.\nc) Many mitigation scenarios consider an "overshoot", assuming that we will exceed the budget and then cool the planet using $\text{CO}_2$ removal technologies. Critically evaluate two biophysical or ecological risks of relying on these technologies for climate stability.