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.

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 Ireland (more frequent heavy rainfall and flooding along the Shannon, earlier flowering, more frequent heat waves, rising sea level on the Irish coast) and explain what each one consists of.

What is a heat wave and how is it defined meteorologically? Explain why, in a warmer climate, heat waves are expected to become more frequent, longer and more intense even if the average temperature only rises by one or two degrees.

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.

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

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.

The IPCC works with emissions scenarios, from the very ambitious to the very high. Explain what a climate scenario is, why science cannot announce a single future temperature, and what role human decisions (energy, economy, demography) play in the range of outcomes.

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.

To determine the causes of recent global warming, a research team analyzes the following empirical observations:\n1. Total solar irradiance oscillates in 11-year cycles but shows a slight downward trend since 1980, while global mean temperature rises continuously.\n2. Major volcanic eruptions cause global thermal drops of $0.2$ to $0.5\ \text{°C}$ lasting between 1 and 3 years, after which warming resumes.\n3. Climate models are only able to reproduce the observed warming from 1970 onwards when they include anthropogenic emissions of greenhouse gases and aerosols.\n4. The isotopic ratio $^{13}\text{C}/^{12}\text{C}$ ($\delta^{13}\text{C}$) in atmospheric $\text{CO}_2$ decreases while the concentration of the radioactive isotope $^{14}\text{C}$ is diluted (Suess effect).\na) Define radiative forcing (in $\text{W/m}^2$) and, based on data 1 and 2, deduce the net sign of solar and volcanic forcing over the last four decades.\nb) Based on data 3, justify why climate simulations demonstrate that anthropogenic forcing is the determining causal factor and not a spurious correlation.\nc) Using data 4, deduce which type of carbon source (volcanic emissions, living plant biomass, or fossil fuels) lacks $^{14}\text{C}$ and is depleted in $^{13}\text{C}$, demonstrating the origin of the excess $\text{CO}_2$.

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.

The average temperature of the island of Ireland has already risen by more than $1.5\ \text{°C}$ since pre-industrial times. Explain what consequences this warming has for the snow cover of Irish mountains and how it changes the flow regime of the rivers that rise in them (more water in winter, less in summer).

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.

Analyze the geographic disparities of warming based on this situation:\nIn recent summers, the Iberian Peninsula has recorded historic heatwaves linked to stalled anticyclonic ridges, while the Arctic was warming at a rate nearly three times higher than the global average (Arctic amplification).\na) Explain the sequence of physical feedbacks (radiative balance, ice/snow albedo, and oceanic absorption) responsible for the Arctic experiencing much more intense warming than the rest of the planet.\nb) The polar jet stream depends on the thermal gradient between the pole and temperate latitudes. Predict how Arctic amplification affects the speed and waviness of the jet, and deduce why this promotes extreme, persistent weather situations (blocking anticyclones, cut-off lows/DANAs) over mid-latitudes.\nc) In a Mediterranean semi-arid basin, analyze the soil-atmosphere feedback loop: predict what happens to the partitioning between sensible heat and latent heat when the soil depletes its moisture in spring, and justify how this phenomenon amplifies extreme summer temperatures.

Explain what carbon sequestration is and name two natural carbon sinks (forests, soils, oceans, peat bogs), indicating how they store carbon and for how long.

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.

Salt marshes and Posidonia seagrass meadows store large amounts of carbon in their sediments ("blue carbon"). Explain why they are especially effective and long-lasting sinks, and discuss what happens to that carbon when a marsh is drained or a meadow is destroyed by anchoring and construction works.

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.

A ski resort at an altitude of $1800\ \text{m}$ in a European mountain range has seen its average natural snow season reduced from 120 to 75 days over the last three decades. To sustain economic activity, the management company plans to build a $200\,000\ \text{m}^3$ water reservoir and install artificial snow cannons to cover $80\ \%$ of the skiable area.\na) Justify why this technical solution can be considered an example of "maladaptation", analyzing its energy balance, its carbon footprint, and the impact on water availability for downstream river ecosystems.\nb) If the winter $0\ \text{°C}$ isotherm rises an additional $150\ \text{m}$ over the coming decades, evaluate the thermodynamic limits of technical snowmaking (wet-bulb temperature) and predict the medium-term viability of the resort.\nc) Design an alternative socioeconomic and ecological adaptation proposal for the valley that replaces snow monoculture, justifying how it diversifies employment and protects alpine biodiversity.

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.

Analyze the following data from two countries ahead of an international climate summit:\n• Country A (developed country): Population: 40 million; cumulative historical emissions (1850-2020): $18\ \%$ of the global total; current per capita emissions: $12\ \text{t}$ of $\text{CO}_2$/year; GDP per capita: $\text{€}\ 42\,000$.\n• Country B (small island developing state): Population: 1 million; cumulative historical emissions: $<0.05\ \%$; current per capita emissions: $1.2\ \text{t}$ of $\text{CO}_2$/year; GDP per capita: $\text{€}\ 3\,000$; its territory suffers from aquifer salinization and irreversible coastal erosion due to sea level rise.\na) Apply the principle of "common but differentiated responsibilities and respective capabilities" to justify what specific financing and mitigation obligations Country A must assume regarding the Loss and Damage Fund for the benefit of Country B.\nb) Country A proposes setting an identical $40\ \%$ cut in emissions for all countries relative to their current levels by 2035. Evaluate the asymmetric impact of this proposal on Country B using climate justice and socioeconomic development criteria.\nc) Design an equitable criterion for sharing the mitigation effort that weights cumulative historical emissions, per capita emissions, and the Human Development Index, justifying why equity is an indispensable requirement for the effectiveness of climate agreements.