General Overview
The seven HS-ESS2 performance expectations address Earth’s dynamic systems at the quantitative level appropriate for high school students who have completed the middle school ESS2 sequence. HS-ESS2-1 asks students to develop a model to illustrate how Earth’s internal and surface processes operate at different spatial and temporal scales to form continental and ocean-floor features. HS-ESS2-2 asks students to analyze geoscience data to make the claim that one change to Earth’s surface can create feedbacks that cause changes to other Earth systems. HS-ESS2-3 asks students to develop a model based on evidence of Earth’s interior to describe the cycling of matter by thermal convection. HS-ESS2-4 asks students to use a model to describe how variations in the flow of energy into and out of Earth’s systems result in changes in climate. HS-ESS2-5 asks students to plan and conduct an investigation of the properties of water and its effects on Earth materials and surface processes. HS-ESS2-6 asks students to develop a quantitative model to describe the cycling of carbon among the hydrosphere, atmosphere, geosphere, and biosphere. HS-ESS2-7 asks students to construct an argument based on evidence for how the co-evolution of Earth’s surface and the life that exists on it have contributed to the conditions for life.
What distinguishes the high school treatment of Earth’s systems from the middle school treatment is above all the emphasis on quantitative modeling and on feedback mechanisms. Middle school students learned that the atmosphere, hydrosphere, geosphere, and biosphere interact. High school students now analyze the specific quantitative mechanisms by which these interactions operate and how changes in one component propagate through the system to affect other components, sometimes amplifying the original change through positive feedbacks, sometimes dampening it through negative feedbacks. Understanding feedback is essential for understanding both the stability of Earth’s climate over geological timescales and its potential for rapid change under anthropogenic forcing. It is also essential for understanding why simple linear extrapolations of current trends are often inadequate descriptions of how complex Earth systems will respond to perturbations.
The primary disciplinary core ideas are ESS2.A (Earth Materials and Systems), ESS2.B (Plate Tectonics and Large-Scale System Interactions), ESS2.C (The Roles of Water in Earth’s Surface Processes), ESS2.D (Weather and Climate), and ESS2.E (Biogeology). The science and engineering practices at the high school level include sophisticated model development, mathematical and computational thinking, data analysis, argument from evidence, and the planning and conducting of investigations. The crosscutting concepts of Energy and Matter, Cause and Effect, Feedback, Stability and Change, and Systems and System Models all appear prominently across the seven standards.
Scope and Sequence
The HS-ESS2 bundle builds directly on the middle school foundation of qualitative system descriptions and develops it into quantitative modeling capability. Middle school students developed a model of the rock cycle showing how energy drives the transformations among rock types. High school students now model the thermal convection in Earth’s mantle that drives plate motion and examine the quantitative relationships between heat flux, viscosity, and convection velocity that determine the rates of tectonic processes. Middle school students evaluated the distribution of earthquakes and volcanoes as evidence for plate boundaries. High school students now model the formation of specific tectonic features at each boundary type and evaluate the timescales over which those features develop. Middle school students described the water cycle driven by solar energy and gravity. High school students now plan and conduct investigations of the specific physical and chemical properties of water that give it its unique role in Earth’s surface processes.
The carbon cycle is the most critical addition at the high school level. While carbon cycling was implicitly present in earlier treatments of ecosystem ecology and weathering chemistry, HS-ESS2-6 requires students to develop a quantitative model that tracks carbon fluxes among the four major reservoirs: atmosphere, oceans, terrestrial biosphere, and geosphere. This quantitative carbon cycle model is the scientific foundation for understanding anthropogenic climate change: human emissions of carbon dioxide from fossil fuel combustion and deforestation represent an additional flux into the atmospheric reservoir that is not balanced by any comparable additional flux out, causing the atmospheric carbon dioxide concentration to rise. Without the quantitative carbon cycle model, climate change is merely an assertion about emissions and temperature. With it, students can understand the causal chain from emission to atmospheric concentration to radiative forcing to temperature change to impacts, and they can evaluate the effectiveness of proposed mitigation strategies in terms of their effect on specific carbon fluxes.
In post-secondary education, the quantitative systems thinking developed in HS-ESS2 provides the foundation for introductory courses in physical geography, oceanography, atmospheric science, geomorphology, and environmental geoscience. It also provides the scientific literacy needed for informed engagement with climate science in policy, economics, journalism, and civic life, making HS-ESS2 one of the most socially consequential components of the high school science curriculum.
What Students Must Understand
Earth’s tectonic features form through the interaction of internal heat-driven processes operating at geological timescales and surface weathering and erosion processes operating at shorter timescales. At divergent plate boundaries, where two plates move apart, magma rises from the mantle to fill the gap, creating mid-ocean ridges and, over tens of millions of years, new ocean basin. At convergent boundaries, where an oceanic plate descends beneath a continental or another oceanic plate in a process called subduction, the descending plate melts, generating magma that rises to form volcanic arcs. The compressional forces also crumple the continental crust to form fold mountain ranges like the Himalayas. At transform boundaries, plates slide horizontally past each other along strike-slip faults, generating earthquakes without significant volcanism. The distribution, scale, and age of features like mountain ranges, ocean trenches, mid-ocean ridges, volcanic arcs, and seamount chains all reflect the specific type and history of plate boundary interactions at each location. The timescales of these processes range from seconds for earthquake rupture to millions of years for the opening of an ocean basin.
Earth’s climate system is maintained by a balance between incoming solar radiation and outgoing thermal radiation, modified by the greenhouse effect. Climate feedbacks are processes that either amplify or dampen the response of the climate system to a perturbation. Positive feedbacks amplify the change: as warming reduces Arctic sea ice, the darker ocean surface absorbs more solar radiation than the reflective ice did, causing additional warming, which further reduces sea ice, creating a reinforcing cycle. The ice-albedo feedback is one of the most important positive feedbacks in the climate system. The water vapor feedback is even stronger: as temperatures rise, more water evaporates into the atmosphere, and water vapor is itself a greenhouse gas, so warming produces more water vapor, which produces more warming. Negative feedbacks stabilize the climate: the Planck response is a fundamental negative feedback in which any warming of Earth’s surface causes it to emit more thermal radiation, reducing the imbalance that caused the warming. Over geological timescales, the carbonate-silicate cycle (the weathering feedback) acts as a thermostat: warmer temperatures accelerate chemical weathering of silicate rocks, which removes carbon dioxide from the atmosphere, reducing the greenhouse effect and cooling the planet back toward equilibrium. Understanding the relative strengths and timescales of positive and negative feedbacks is essential for understanding both Earth’s long-term climate stability and its potential sensitivity to rapid perturbations.
The global carbon cycle tracks the movement of carbon among four major reservoirs: the atmosphere (approximately 850 gigatons of carbon as carbon dioxide), the terrestrial biosphere (approximately 2,000 gigatons in living biomass and soil organic matter), the ocean (approximately 38,000 gigatons, primarily as dissolved inorganic carbon), and the geosphere (approximately 50 to 100 million gigatons in sedimentary rocks, coal, petroleum, and natural gas). Carbon moves among these reservoirs through photosynthesis (atmosphere to biosphere), respiration (biosphere to atmosphere), ocean-atmosphere gas exchange (atmosphere to and from ocean), weathering (geosphere to ocean and atmosphere), volcanic outgassing (geosphere to atmosphere), and organic matter burial (biosphere and ocean to geosphere). The geological carbon cycle operates over millions of years and maintains atmospheric carbon dioxide at habitable levels through the weathering feedback. The biological carbon cycle operates over years to decades and is responsible for the seasonal oscillation in atmospheric carbon dioxide visible in the Keeling Curve. Human fossil fuel combustion currently adds approximately 10 gigatons of carbon per year to the atmosphere, roughly a hundred times the natural volcanic outgassing rate, at a rate that far exceeds the absorption capacity of natural sinks.
The properties of water give it a uniquely important role in Earth’s surface processes. Water has an unusually high heat capacity, meaning it can absorb and release large amounts of thermal energy with relatively small changes in temperature. This makes the ocean an enormous heat reservoir that moderates climate by absorbing excess heat in summer and releasing it in winter, dampening the seasonal temperature swings experienced by adjacent land masses. Water expands upon freezing, which is unusual among substances and critical for aquatic ecosystems: ice floats on liquid water, insulating the liquid water below from further freezing and allowing aquatic life to survive winter. Water is an excellent solvent, capable of dissolving a wide range of ionic and polar substances, which makes it the medium of virtually all biological chemistry and the primary agent of chemical weathering. The water molecule’s polarity gives rise to hydrogen bonding, which accounts for water’s high surface tension, its relatively high boiling point for its molecular weight, and its ability to be drawn up through the narrow tubes of plant xylem against gravity through capillary action.
Key vocabulary includes: divergent boundary, convergent boundary, transform boundary, subduction, volcanic arc, fold mountain, isostasy, thermal convection, mantle plume, albedo, radiative forcing, feedback, ice-albedo feedback, water vapor feedback, Planck response, carbonate-silicate cycle, carbon cycle, gross primary production, net primary production, respiration, weathering flux, volcanic outgassing, organic burial, Keeling Curve, heat capacity, hydrogen bonding, and capillary action.
Lesson Ideas and Activities
A tectonic landform model development investigation addresses HS-ESS2-1 by asking students to construct a physical or digital model that shows how specific landforms develop over geological time at each type of plate boundary. Using materials such as foam layers, clay, and sandboxes with water, student groups model one assigned boundary type and produce both the model itself and a diagram showing the spatial relationships among the features that develop over 10 million, 50 million, and 100 million years of plate interaction. Groups present their models to the class. The teacher facilitates a synthesis discussion connecting all three boundary types and addressing the temporal scales at which features at each boundary type develop. This activity develops the ability to use models to represent processes operating at scales too large and too slow to observe directly, which is the central modeling challenge of plate tectonics education.
A climate feedback analysis investigation builds on published climate feedback research to develop understanding of HS-ESS2-2 and HS-ESS2-4. Students are given a graphical dataset showing several key climate variables over the past 800,000 years from Antarctic ice cores: temperature anomaly, atmospheric carbon dioxide concentration, atmospheric methane concentration, and ice volume. Students identify periods of rapid warming and cooling and examine the behavior of each variable before, during, and after the temperature transitions. They identify which variables lead and which lag temperature change, and construct hypotheses about which are forcing the change and which are responding as feedbacks. They then compare their hypotheses to published scientific analysis of the same data. This investigation develops the ability to analyze complex multi-variable datasets for causal relationships and feedback structure, which is the central analytical skill of quantitative climate science.
A thermal convection modeling investigation provides the physical foundation for understanding plate motion and deep Earth dynamics as required by HS-ESS2-3. Students observe convection directly using a transparent tank of heated water with food coloring, noting the cells of rising hot fluid and sinking cool fluid. They then apply the viscosity concept: add corn syrup to the water and observe how increasing viscosity slows convection. Finally, students examine published estimates of mantle viscosity and thermal gradient and use scaling arguments to estimate the velocity of mantle convection. The disconnect between laboratory convection, which operates in seconds, and mantle convection, which operates over millions of years, directly illustrates why the mantle can be solid on seismic timescales but flow plastically on tectonic timescales. This investigation develops the dimensional reasoning and scaling thinking that are essential for applying laboratory physics to geological processes.
A quantitative carbon cycle modeling investigation is the core activity for HS-ESS2-6. Students work with a simplified version of the IPCC carbon cycle model, which they can implement in a spreadsheet. They set up the four major reservoirs, specify the flux rates among them (using published estimates from the IPCC Fifth Assessment Report), and run the model forward in time under different scenarios: natural variability only, addition of fossil fuel emissions at current rates, emissions doubled, emissions reduced to zero. Students compare the atmospheric carbon dioxide trajectories under each scenario and analyze: how long does it take for atmospheric carbon dioxide to stabilize after emissions cease? Why does stabilization take so long? What does this imply about the temperature trajectory after emissions peak? This investigation makes the dynamics of the carbon cycle quantitatively concrete and directly connects to the climate change policy questions addressed in HS-ESS3.
A water properties investigation addresses HS-ESS2-5 through carefully designed physical experiments. Students measure the specific heat capacity of water and compare it to sand and soil, then reason about the implications for coastal versus continental climates. They observe the density anomaly of water near the freezing point by carefully measuring the density of water at 1 degree Celsius, 4 degrees Celsius, and 10 degrees Celsius and finding that water is densest at 4 degrees rather than at the freezing point. They investigate capillary rise in tubes of different internal diameters and connect the phenomenon to both plant water transport and the movement of water through soil. Finally, they measure the solubility of a salt and a nonpolar substance in water and discuss why water is an excellent solvent for ionic and polar substances but not for nonpolar ones. Each experiment is followed by a discussion connecting the measured property to a specific Earth surface process that depends on it.
A biogeology evidence analysis investigation addresses HS-ESS2-7 by having students examine datasets showing the coevolution of Earth’s surface environment and life over the past 4 billion years. Students analyze: the evidence for the timing and cause of the Great Oxidation Event approximately 2.4 billion years ago, when atmospheric oxygen first rose to significant levels as a result of cyanobacterial photosynthesis; the evidence for Snowball Earth events in the Neoproterozoic, when Earth may have been almost entirely glaciated; the evidence connecting the rise of land plants in the Devonian to a dramatic decrease in atmospheric carbon dioxide and the first major coal-forming forests; and the evidence for the rapid diversification of complex animal life at the beginning of the Cambrian, which may be connected to rising oxygen levels and changing ocean chemistry. Students construct a timeline argument: “Life has not merely adapted to Earth’s changing environment; it has actively modified Earth’s environment in ways that created new conditions for further biological evolution.”
Common Student Misconceptions
A common misconception about plate tectonics at the high school level is that the driving force is primarily convection cells in the mantle that the plates ride like conveyor belts. This simplified model was widely taught for decades and is found in many textbooks, but modern geodynamics research has established that the primary driving force of plate motion is slab pull: the gravitational sinking of cold, dense oceanic lithosphere at subduction zones drags the rest of the plate with it. Ridge push, the gravitational force produced by the elevated ridge flanked by downward-sloping ocean floor, is a secondary driver. Mantle convection is more a consequence of subducting slabs disturbing the mantle than the primary cause of plate motion. Teaching the current understanding, while acknowledging that the details are still actively researched, develops a more accurate model and avoids the misleading image of plates passively carried by convection currents beneath them.
A second misconception is that positive feedbacks in the climate system inevitably lead to runaway warming. Students who learn about the ice-albedo and water vapor positive feedbacks sometimes conclude that any warming will amplify itself without limit, producing catastrophic temperature increases. In fact, the climate system also contains important negative feedbacks, particularly the Planck response, which is sufficiently strong to prevent runaway warming under the perturbations currently being imposed by human greenhouse gas emissions. The relevant scientific question is not whether the climate will run away, which is not predicted by current models for anything short of extreme forcing, but how sensitive the climate system is to a given level of forcing: that is, how many degrees of warming will result from a doubling of atmospheric carbon dioxide. The current best estimate of equilibrium climate sensitivity is 2.5 to 4 degrees Celsius per doubling, a range that reflects genuine scientific uncertainty about feedback strengths, not a runaway scenario.
A third misconception about the carbon cycle is that carbon dioxide removed from the atmosphere by the ocean is permanently sequestered and unavailable to affect future climate. The ocean is not a one-way sink for carbon but an active reservoir that exchanges carbon with the atmosphere through gas exchange processes driven by temperature and biological productivity. As the ocean warms, its solubility for carbon dioxide decreases, meaning it will absorb a smaller fraction of future emissions than it has of past emissions. The deep ocean does store carbon on thousand-year timescales, but changes in ocean circulation can return this stored carbon to the surface and eventually to the atmosphere. The permanence of ocean carbon storage is timescale-dependent: permanent on human timescales for deep sequestration, but not permanent on geological timescales.
A fourth misconception is that climate models are too uncertain to be useful for policy. Students who encounter the range of projections in IPCC reports sometimes interpret the uncertainty range as evidence that scientists know nothing useful about future climate. In fact, uncertainty ranges in climate projections represent the range of outcomes under a range of emission scenarios and climate sensitivities, not complete ignorance. The climate science community has correctly predicted many observable consequences of greenhouse gas forcing, including the pattern of atmospheric warming (troposphere warming, stratosphere cooling), the differential warming of land versus ocean, the warming of the Arctic at approximately twice the global mean rate, and the increase in global mean sea level. The existence of remaining uncertainty about exact magnitudes does not negate the robust predictions about the direction and general scale of climate change under continued greenhouse gas emissions.
A fifth misconception is that Earth’s internal heat is primarily a relic of its initial formation and will eventually cool to the point where tectonic activity ceases. While residual heat from accretion and differentiation does contribute to Earth’s internal heat budget, the primary source of ongoing heat generation is the radioactive decay of uranium, thorium, and potassium in the mantle and crust. This radiogenic heat production decreases with time as the radioactive elements decay, meaning that Earth’s interior is indeed slowly cooling and tectonic activity will eventually cease, but on timescales of billions of years in the future. Earth’s interior is sufficiently hot at present to sustain vigorous convection for the foreseeable future of life on Earth. Mars, which is smaller and cooled faster, has already reached the state of tectonic inactivity toward which Earth is slowly moving.
A sixth misconception about the carbon cycle is that natural carbon sources like volcanoes or ocean outgassing are as significant as or more significant than human fossil fuel combustion. Volcanic outgassing contributes approximately 0.1 to 0.15 gigatons of carbon per year to the atmosphere, while human fossil fuel combustion contributes approximately 9 to 10 gigatons per year. Human emissions are thus approximately 60 to 100 times larger than volcanic emissions. Furthermore, on geological timescales, volcanic outgassing is approximately balanced by the removal of carbon through chemical weathering and marine carbonate formation, making the geological carbon cycle nearly neutral. Human emissions represent an additional, unbalanced flux that has no equivalent negative feedback on human timescales. Students who misunderstand the relative magnitudes of natural and anthropogenic carbon fluxes are unable to reason accurately about the attribution of current atmospheric carbon dioxide increases to human activities.
Assessment Questions
Develop a model of how a subduction zone produces the geological features observed at a convergent plate boundary. Your model should include: the mechanism of subduction, why oceanic lithosphere subducts rather than continental lithosphere, the process by which magma is generated above the subducting slab, the formation of the volcanic arc and the back-arc basin, and the formation of an accretionary prism. Identify the energy source driving each component of the system and the approximate timescale over which each feature develops.
The following data shows atmospheric temperature anomaly and carbon dioxide concentration from the EPICA Dome C ice core over the past 400,000 years. Identify three periods of rapid warming and three periods of rapid cooling in the temperature record. For each transition, describe the behavior of carbon dioxide relative to temperature. Does carbon dioxide appear to lead or lag temperature change? What implications does this timing relationship have for our interpretation of whether carbon dioxide is a forcing or a feedback during these glacial-interglacial cycles?
Describe the ice-albedo feedback and the water vapor feedback in the climate system. For each feedback: explain the mechanism by which the initial perturbation is amplified, identify whether it is a positive or negative feedback, and estimate the approximate strength of the feedback relative to the initial forcing. Why does the existence of multiple positive feedbacks not lead to runaway warming under current conditions?
Construct a quantitative carbon cycle model with four reservoirs: atmosphere, ocean, terrestrial biosphere, and geosphere. For each reservoir, estimate its current size in gigatons of carbon. For each of the major fluxes among reservoirs, identify the process, estimate its magnitude, and identify whether it is primarily a physical, chemical, or biological process. Using your model, estimate what fraction of current human fossil fuel emissions remains in the atmosphere after one year, after 10 years, and after 100 years, taking into account the absorption capacity of the ocean and terrestrial biosphere. What does this analysis tell us about why atmospheric carbon dioxide continues to rise even if human emissions were substantially reduced tomorrow?
The Great Oxidation Event approximately 2.4 billion years ago saw atmospheric oxygen rise from essentially zero to approximately 2 percent. Construct an argument, supported by evidence, for how cyanobacterial photosynthesis drove this transition. Your argument should address: what evidence tells us that oxygen was essentially absent before 2.4 billion years ago, what evidence shows that oxygen rose at approximately 2.4 billion years ago, what mechanism caused oxygen to accumulate at that time rather than earlier, and how the rise in oxygen affected both the geochemical and biological environment.
Compare the rates at which natural processes remove carbon dioxide from the atmosphere with the rate at which human fossil fuel combustion adds carbon dioxide. What does this comparison tell us about the timescale over which the atmospheric carbon dioxide concentration will return to pre-industrial levels if human emissions cease? What geological processes might eventually restore the pre-industrial carbon dioxide concentration, and over what timescale would they operate?
Design an investigation to determine whether water has a higher heat capacity than sand or soil. Specify the materials, measurements, and controls you would use. Explain how the results of your investigation connect to the observed difference in temperature variability between coastal and inland locations at the same latitude. What other properties of water contribute to its role as a moderator of climate in coastal regions?