2: Back to the Present and Future of Climate Change

Back to the Recent Past and Present

Let's return to more recent human history and the anthropogenic forcing of our climate.  Figure \(\PageIndex{1}\) shows atmospheric CO₂ levels since 1000.  Zoom in on the steep rise in CO₂ that started around 1760 with the Industrial Revolution

Figure \(\PageIndex{1}\): An interactive graph of atmospheric CO₂ over more recent times.

Zoom in on the steep rise in CO₂ that started around 1760 with the Industrial Revolution. Figure \(\PageIndex{1}\): Interactive graph of atmospheric CO₂ vs time over the last 1000 years. Historical CO₂ record from the Law Dome DE08, DE08-2, and DSS ice cores. Credits: D.M. Etheridge, L.P. Steele, R.L. Langenfelds, R.J. Francey and the Division of Atmospheric Research, CSIRO, Aspendale, Victoria, Australia. 2 Degrees Institute. https://www.2degreesinstitute.org/. Also, https://www.co2levels.org/

Orbital mechanics cannot explain the planet's warming over the brief (geological) period since the Industrial Revolution. Neither can volcanic activity, changes in solar activity, changes in land use (for example, deforestation that slows photosynthesis and CO₂ removal from the atmosphere), or even aerosols released by burning fossil fuels (which would decrease global temperature by increasing the reflection of sunlight). Click on Figure \(\PageIndex{2}\) to see an animated explanation of which factors best explain the global temperature increase since 1880. The results are clear: It's us!

Figure \(\PageIndex{2}\): Factors contributing to global warming based on NASA data and NASA's Goddard Institute for Space Studies (GISS) climate models.

Unfortunately, other greenhouse gases have also risen since 1975, as shown in Figure \(\PageIndex{3}\).

Figure \(\PageIndex{3}\): Rise in greenhouse gases since 1975. https://www.co2.Earth/annual-ghg-index-aggi

Note the zig-zag nature of the CO₂ curve. The curve dips slightly in the summer, when plants in the Northern Hemisphere actively remove CO₂. Increasing methane now accounts for up to 20% of the warming observed. Figure \(\PageIndex{4}\) shows an interactive graph of a very worrisome rise in atmospheric methane over time.

Figure \(\PageIndex{4}\): Interactive graph of atmospheric CH₄ vs time over the last 1000 years. 2 Degrees Institute. https://www.2degreesinstitute.org/

Humans unequivocally cause present-day warming by burning fossil fuels.

Past Climate Anomalies

Several dramatic but short-lived (in geological time) climate changes have punctuated recorded human history. Let's look at two, mostly to equip you to address climate skeptics. They also show how sensitive our climate is to subtle changes.

The Little Ice Ages 

Actual and proxy temperature records show a mild period in Europe from around 950-1100, followed by colder weather, especially from 1450 to 1850. The latter period is called the "Little Ice Age," although the Northern Hemisphere ice shield did not significantly expand. It was especially cold worldwide in 1816 when much of the world experienced a "year without summer." The 1816 effect had a clear cause: the eruption of Mount Tambora in Indonesia on April 10, 1815.

In addition to this identifiable influence in 1816, a cool period was reported in the Northern Hemisphere from about 1800 to 1820, beginning before the Tambora eruption. Also, a low period of solar irradiance, the Dalton Minimum, occurred from 1790 to 1860. Proxies for solar activity in the 1600s also show small drops in solar irradiance, as discussed below. The dip in global average temperatures following the Medieval Warm Period is shown in Figure \(\PageIndex{5}\).

Figure \(\PageIndex{5}\): Dip in global average temperatures following the Medieval Warm Period, By RCraig09 - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/inde...curid=87832845

Literature and art have captured modern climate change. One example is a painting showing "Ice Fairs" on the Thames in London, shown in Figure \(\PageIndex{6}\).

Figure \(\PageIndex{6}\):https://commons.wikimedia.org/wiki/F...enell.jpgdfdfd

Many factors probably contributed to the Little Ice Age, including a drop in solar irradiance. A newer explanation has also been proposed. Marine records show that waters near Greenland and the Nordic seas were warmer because the Atlantic Meridional Overturning Circulation (AMOC) strengthened. This would have caused Arctic ice loss in the late 1300s and 1400s, cooling the water and diluting its salinity because ice, when it crystallizes with a tetrahedral hydrogen-bonded coordination of water, excludes salt. This would have collapsed the AMOC and its heat transfer to the northern waters, leading to rapid and prolonged cooling. An analogous strengthening of the AMOC was observed between 1960 and 1980, which was attributed to a long-duration high-pressure system over Greenland. A similar event might have kick-started the Little Ice Age. Tree rings indicate higher solar irradiance before the Little Ice Age, which may be associated with the initial strengthening of the AMOC.

The Little Ice Age also affected China and may have contributed to crop failure in 1644, when the Ming Dynasty fell. An Arctic hurricane in 1588 helped destroy the Spanish Armada. The Great Fire of London in 1666 followed a very dry summer after an exceptionally cold winter. Food production was severely disrupted, potentially leading to significant social change in Europe and elsewhere as the Plague shattered societal and cultural norms.

The 1816 eruption of Mount Tambora in present-day Indonesia greatly worsened the cooling. The ash and SO₂ aerosols blocked solar irradiance. Droughts, floods, cholera epidemics, famine, and migration from Europe to the US and East to West partly arose from this event.  

One of the worst times to be alive: 536

Historians report that in 536 AD, parts of Europe, the Middle East, and Asia experienced 24 hours of darkness for up to 18 months.  Summer temperatures plummeted. Famines occurred for a few years after. It snowed in China in the summer. The worst effects were in the Northern Hemisphere, but they were worldwide. It was probably the most pronounced cooling in the last 2000 years. To make matters worse, a pandemic erupted around 541 and spread from southern Asia to northern Europe. It had a huge effect on the Byzantine Empire and is known as the Justinian (bubonic) Plague after the Byzantine emperor. Crop failures, trade expansion, and an influx of rodents driven by the cold temperatures could have led to and exacerbated the plague.

This second, more severe example of cooling was shorter-lived on a geological timescale. Temperatures fell in the summer by about 1.5-2.5⁰C. A "smoking gun" has been linked to this cooling: a volcanic explosion in Iceland. In addition, another eruption occurred in 540, which lowered the temperature by 1.5-2.50C, and another in 547. The combined effects of climate change and the plague led to a significant economic decline in Europe. Signs of airborne lead in the ice at 640, originating from silver mining, suggest a recovery in economic growth. You should ask yourself how the modern world copes with such an occurrence. 

Solar Activity and Climate Change

We have discussed how orbital forcing kick-started each recurring ice age in the Pleistocene. Some effects of the change in solar activity independent of the sun's orbit have been noted above. Specifically, we have shown that it cannot account for present warming. We present a series of graphs from the NOAA (National Oceanic and Atmospheric Administration) in the collective Figure \(\PageIndex{7}\) below to show the actual change in solar activity over recent times. Comments are shown at the bottom of each graph.

Figure \(\PageIndex{7a}\): Actual change in solar activity over recent times. Dark red to the right are actual temperature differences

The maximum % spread between the lowest and highest is very small. Such a small change shouldn't have such dramatic effects on climate unless sustained, as it was from around 1630-1700. Hence, this decline in solar activity likely contributed to the Little Ice Age. The regular rise and fall (spikes) are associated with the 11-year sunspot cycle activity. Note that the rise in average temperature since 1910 (shown in red) cannot be accounted for by a change in solar activity.  Figure \(\PageIndex{7b}\) below shows the irradiance over time with the Maunder and Dalton minimums labeled.

Figure \(\PageIndex{7b}\):  Irradiance over time with the Maunder and Dalton minimums

The above graph shows that the irradiance decreased by about 0.06% (although other values have been reported as high as 0.22%) during the Maunder Minimum, which occurred in the Little Ice Age. The average decrease in terrestrial temperatures was 1.0- 2.0 °C.  

Figure \(\PageIndex{7c}\) below shows a best-fit average temperature difference (dark red line) compared to the 1800-1960 average. 

Figure \(\PageIndex{7c}\): best-fit average temperature difference (dark red line) compared to the 1800-1960 average.

 Note that the averages are lower during the Little Ice Age, with the lowest values and the lowest spike temperatures near and in the Maunder Minimum.  A drop of just 1°C had profound effects on Europe's climate.

Figure \(\PageIndex{7d}\) below shows irradiance change from 1980 to 2020 through 4 solar cycles.  

Figure \(\PageIndex{7d}\): Irradiance change from 1980 to 2020 through 4 solar cycles.

The graph shows the 11-year repeat of sunspot activity and resulting solar irradiance. Although activity dropped in 2020 (not shown in the figure), it was the second-warmest year on record since 1880.

Finally, Figure \(\PageIndex{7e}\) below shows irradiance over the last 10,000 years (from the end of the last Ice Age) until 0 CE.

Figure \(\PageIndex{7e}\): Irradiance over the last 10,000 years

This graph does not show the effects of orbital climate forcing. Rather, it shows that solar activity did not change significantly for the 10,000 years before 0 CE.

So the climate warming we see now can't be attributed to changes in solar irradiance.  We have injected a massive amount of CO₂, approximately 1.5 trillion tons, into the atmosphere since the Industrial Revolution from using fossil fuels. Not all of it remains in the atmosphere, but enough to raise CO2 to levels not seen in 3 million years. Based on the relationship between CO₂ and temperature across the ice ages, science can predict when conditions might exist to initiate and propagate the next ice age. The data arising from these models, illustrated in Figure \(\PageIndex{7f}\), show how much incoming solar radiation (insolation) must arrive at the Earth (watts/m²) to trigger the next ice age.

Figure \(\PageIndex{7f}\): Incoming solar radiation required to trigger the next ice age.

As shown on the left side of the figure, if CO₂ were 280 ppm (typical of peaks in past interglacial periods), it would require repeated drops in insolation below the threshold of about 455 watts/m² (red line) to initiate glaciation. As of June, 2026, we are at 430 ppm and rising. If it rises to 450 ppm, as it surely will without carbon capture, it would require much less insolation to produce an ice age, since the greenhouse effect of higher CO₂ would warm the atmosphere. The right graph shows little chance of another ice age without large, sustained volcanic activity or an asteroid impact that would block solar radiation.

Summary of Climate Change Causes and Effects Since 900 AC

Figure \(\PageIndex{8}\) summarizes possible contributions to temperature change over the last 1000 years. Note again that our current warming (1950 onward on the right side of the panels) is attributable solely to greenhouse gases (GHGs). One panel shows changes in land use. This has caused a decline in temperature since 1800. Deforestation and other land-cover changes cause this effect by increasing the reflection of incident solar radiation back into space. This effect is greater in winter if the altered land is snow-covered. Deforestation would also reduce CO₂ uptake (via photosynthesis), thereby raising atmospheric temperature. That component is added to the GHG panel. 

Figure \(\PageIndex{8}\): Simulated northern hemisphere temperature changes, smoothed with an 11-year running mean, relative to AD 950–1250. Owens et al. J. Space Weather Space Clim. 2017, 7, https://doi.org/10.1051/swsc/2017034. Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0).

The black line in the top panel shows the observed instrumental Northern Hemisphere temperature variations with their associated uncertainties (Morice et al., 2012), which match the simulations well. The bottom panel shows a simulation with no changes in radiative forcing. This quantifies the magnitude of natural internal variability in the simulations without changes in forcings. Note periodic but short-term dips in temperature due to volcanic activity. Grey-shaded regions represent the three Little Ice Age (LIA) periods, with the darkest interval (LIA3) most closely matching the Maunder Minimum (MM), a period of extremely low solar activity from 1650 to 1715.  Warming since the Industrial Revolution (far right) is due to emissions from burning fossil fuels. 

Climate Justice: The Emitters and the Affected

We present a series of graphs in Figure \(\PageIndex{9}\) below, taken from CO₂ Emissions - Our World in Data, to show which countries have emitted the most CO₂ in the past and now. In a just world, those countries that have emitted the most should move swiftly and forcefully not only to reduce emissions but also to support other countries' transitions to clean fuels and to help them with climate change mitigation and adaptation. We don't wish to demonize the fossil fuel industry and those who work in it. The use of fossil fuels, which are high-energy, high-density, and cheap fuel (because of historically massive subsidies), has lifted billions out of poverty over time. Until recently, we had no alternative to fossil fuels. Most did not realize how significantly fossil fuel use would affect our present and future climate, human health, and the entire biosphere.  Yet, we can't just stop using fossil fuels without inflicting great economic pain on those who can least afford it. To help those currently suffering and those who will suffer most in the future, and to help ourselves, our children, and our grandchildren, we must move away from using fossil fuels as soon as possible through a planned drawdown of their use.

Above: The dip in total world emissions in 2020 was due to the COVID-19 pandemic. Unfortunately, the rise has resumed. China is now the largest net emitter, but emissions in the US and EU are declining. India is on the rise, and if it follows a similar economic path as China, which it needs to lift many out of poverty, it will come with a huge cost in CO₂ emissions unless it can jumpstart their conversion to clean fuels. The world needs to help.

Above: Although China is the largest net emitter, the US and Australia are the largest emitters per capita, though that is declining.

The US still leads the world in total CO₂ emitted since the Industrial Revolution. We also have the greatest GDP. In 2022, Pakistan suffered tragic flooding exacerbated by climate change. Up to a third of the country was underwater. In a just world, the biggest emitters would aid the rest of the world.

Above: Inequality is evident in this graph as the wealthiest people (high and upper-middle income) collectively contribute 86% of CO₂ emissions

Figure \(\PageIndex{9}\): CO₂ emissions by country and income since the Industrial Revolution. Our World in Data.  Creative Commons BY license. 

Future Projections

We know the science and the consequences if we choose not to act or act in ways insufficient to meet the challenges of climate change. It is one of the most difficult challenges we have faced as a species. It requires sacrifice and united action for the common good. The benefits of our choice are mostly in the future and for future generations. We have models that predict future warming.  As climate models become more sophisticated, as we have more data, and as policies evolve, future scenarios for likely warming change as well.  There are 3 main types of models important to this chapter.  The newest one (June 2026) is most plausible given present trends.  The first two (RCP and SSP) are shown in blue inset boxes with a clickable link for more background.

We mentioned in 31.1A that a new value for climate sensitivity, which gave rise to a rise of 4.8 °C (8.6 °F) for a doubling of CO₂ was determined by Hansen et al. (2023).  This value has startling implications.  If this new value holds, the IPCC/UN call to keep warming to 1.5°C above the pre-industrial revolution average is now impossible.  We are already there or will be there in a few years.  We are unlikely to make the changes required to keep warming below 2°C before 2050. The reasons for our inaction are politicians who continue to support the fossil fuel industry and who block significant legislation to act now, religious institutions and leaders who don't make climate action a sacred duty, and the fossil fuel industry that continues to expand fossil fuel exploration and mislead the public as to the causes of climate change and action to address it.  Those who say we can't afford to act never mention the much greater cost of inaction!

Around 2020, the IPCC shifted away from the RCP label to the Shared Socioeconomic Pathways (SSPs) model.    

Both of these scenario types rely on the energy imbalance (W/m²).   We discussed in Chapter 32.1: A that Earth's Energy Imbalance (EEI) rose from about +0.6 Watts/m² in 2000 to +0.9 W/m² from 2005 to 2019, and to +1.0 W/m² in 2021. The trend is, unfortunately, upward, as shown in Figure \(\PageIndex{13}\).

Figure \(\PageIndex{13}\):  Earth's Energy Imbalance since 2000.  12-month running-mean of Earth’s energy imbalance from CERES satellite data normalized to 0.71W/m2 mean for July 2005–June 2015 (blue bar).   James E Hansen, Makiko Sato, Leon Simons, Larissa S Nazarenko, Isabelle Sangha, Pushker Kharecha, James C Zachos, Karina von Schuckmann, Norman G Loeb, Matthew B Osman, Qinjian Jin, George Tselioudis, Eunbi Jeong, Andrew Lacis, Reto Ruedy, Gary Russell, Junji Cao, Jing Li, Global warming in the pipeline, Oxford Open Climate Change, Volume 3, Issue 1, 2023, kgad008, https://doi.org/10.1093/oxfclm/kgad008.  Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/)

The projected value of 1.36 (red bar) for the 2020s bodes poorly for our ability to reverse climate change.  The large change that began around 2015 likely stems from decreased atmospheric sulfate emissions (required by legislation) from ships in the northern Atlantic and Pacific oceans and from coal burning in China.  These decreases in atmospheric aerosols, which cause cardiovascular, pulmonary, and cancer diseases, paradoxically increase global warming.  Sulfate aerosols reflect solar radiation.  They also form clouds, which do the same.  Both cool the planet.  Aerosols that contribute to some of the world's worst pollution in South Asian cities have helped keep temperatures from rising to dangerous levels.

Recent Updates: May, 2026 

3.  Coupled Model Intercomparison Project Phase 7: CMIP7

Van Vuuren, D. P. et al.: The Scenario Model Intercomparison Project for CMIP7 (ScenarioMIP-CMIP7), Geosci. Model Dev., 19, 2627–2656, https://doi.org/10.5194/gmd-19-2627-2026, 2026.

As our climate models improve and the world adopts more green energy, we need to revisit the RCP and SSP models described above.  Van Vuuren et al.'s analyses suggest that the most dire 8.5 model is no longer needed as a possible worst-case scenario.  This shows the progress the world has made toward fighting climate change.  Some in the media suggest that the 8.5 model was developed to scare the public and policymakers into climate action.  Its actual use was to set an upper limit on the theoretical impacts of climate drivers and conditions, a feature any predictive model would use.  For example, financial planners use mathematical analyses called Monte Carlo simulations to model uncertainty.  1000s to millions of simulations are used in calculations, with different random input values to generate a distribution of outcomes and determine the most and least likely scenarios.  

The latest model is the CMIP7 (Coupled Model Intercomparison Project, Phase 7), organized under the World Climate Research Program (WCRP).  In the CMIP system, modelers run their own models (based on the physics, chemistry, and biology of the entire biosphere) using the same protocols, climate forcings, and data sets to develop scenarios that can be compared and tested.  It's like running different experiments around the world, using the same data with different models. The project analyzes an astonishing amount of data. 

The CMIP7 models use newly defined scenarios, named after emission trends, not climate forcing (W/m²) or socio-economic-political drivers:

• High emission scenario: A scenario with emissions as high as judged to be plausible, based on assuming developments that include a rollback of current mitigation policies. This scenario is expected to result in forcings below SSP5-8.5.

• High-to-Low emission scenario: A scenario that follows approximately the same emissions pathway as the High, but changes course in the second half of the century, applying strong mitigation measures to reach net zero CO₂ emissions by 2100.

• Medium emission scenario: A middle scenario exploring the consequences of extending current policies and trends into the future.

• Medium-to-Low emission scenario: A scenario exploring a delayed increase in mitigation efforts, short of the Paris temperature goal but achieving net-zero CO₂ emissions by the end of the century, with a period of net negative CO₂ emissions thereafter to achieve 1.5 °C on a multi-century timescale.

• Low-emission scenario: The low-emission scenario is designed to be consistent with the goal of keeping warming below 2 °C, without returning to 1.5 °C before the end of the century. The scenario extends beyond 2100 under an emissions trajectory that leads to a gradual decrease in warming afterward.

• Very Low emission scenario: The Very Low emission scenario is designed to keep the temperature level as low as plausible given feasibility constraints. This scenario is thus relevant for the low end of the Paris range (staying as close as plausible to 1.5 °C at the time of peak warming and limiting warming to 1.5 °C by the end of the century).

• Low-to-Negative emission scenario: The last trajectory shows a higher overshoot of the 1.5 °C goal, followed by stringent climate policies that result in net-negative greenhouse gas emissions, returning to lower warming levels and thus supporting research into the reversibility of climate outcomes and their impacts.

Figure \(\PageIndex{14}\) below shows the CMIP7 model color-coded emission trends (a, left) and the resulting warming (b, right)

Figure \(\PageIndex{14}\): Proposed scenarios for CMIP7 ScenarioMIP, showing 

(a) GHG emissions pathways as a function of time for each of the proposed scenarios (based on GWP-100) and 

(b) The associated global average temperature outcomes using the probabilistic FaIR ensemble used in IPCC AR6 (Smith, 2025; Smith et al., 2024).

The shaded area for emissions shows ±8 GtCO₂ around the expected marker scenario value, while for temperature outcomes, it shows the 33–67 percentile range of the distribution for the same expected marker scenario. Data for the historical period are available in Nicholls et al. (2025a). Scenarios are (H) High, (HL) High-to-Low, (M) Medium, (ML) Medium-to-Low, (L) Low, (LN) Low-to-Negative, and (VL) Very Low. The final emission trajectories will depend on the finalized IAM runs but are expected to be roughly consistent with the illustrations provided here. The final temperature outcomes will be known only upon completion of Earth system model experiments and, for this phase of ScenarioMIP, will also include the effects of carbon cycle feedbacks. Models are requested to run the scenarios for the period 2025–2150; the textured regions for the 2100–2150 period (AD) indicate that these emissions and forcings are defined in the extension protocol.

It's not just about carbon:  The Sulfur Cycle

We have discussed the carbon and nitrogen cycles. The sulfur cycle also affects Earth's climate. As described above, sulfur aerosols (such as those produced by SO₂) can locally lower atmospheric temperatures by reflecting incoming solar radiation.  Of course, they also harm the environment and human health by contributing to acid rain.  Reduced sulfur-containing compounds, such as H₂S (hydrogen sulfide), (CH₃)₂S  (dimethyl sulfide), and DMSP (dimethylsulfoniopropionate), are also part of the cycle.  A simplified version of the sulfur cycle is shown in Figure \(\PageIndex{15}\)  below. 

Figure \(\PageIndex{15}\):  The sulfur cycle.  Elmer Robinson and Robert C. Robbins.  Bantle, CC0, via Wikimedia Commons. https://commons.wikimedia.org/wiki/F...o_Enxofre).png.   Creative Commons CC0 1.0 Universal Public Domain Dedication

Ocean phytoplankton, other microorganisms, and some plants release DMS into the atmosphere. These can form aerosols that help cool the climate and transfer sulfur to the land. The structures of DMS and DMSP, along with the enzymes involved in their formation, are shown below in Figure \(\PageIndex{16}\).

Figure \(\PageIndex{16}\): Formation of Methylsulfide. After Carrión, O., Curson, A., Kumaresan, D. et al. A novel pathway producing dimethylsulphide in bacteria is widespread in soil environments. Nat Commun 6, 6579 (2015). https://doi.org/10.1038/ncomms7579.  

Panel A shows the DMSP cleavage pathway, catalyzed by DMSP lyase enzymes (Ddd) found in some bacteria.  Panel B shows the pathway in Pseudomonas deceptionensis M1^T.  Methionine (Met) is converted to MeSH using Met gamma lyase enzyme (MegL) in P. deceptionensis M1^T.  MeSH is methylated to DMS by the enzyme MddA (a membrane protein methanethiol S-methyltransferase) using Ado-Met as the methyl donor.  

DMSP can be synthesized from methionine via a variety of pathways, as shown in Figure \(\PageIndex{17}\).

Figure \(\PageIndex{17}\): DMSP biosynthesis genes, enzymes, and pathways.  Wang, J., Curson, A.R.J., Zhou, S. et al. Alternative dimethylsulfoniopropionate biosynthesis enzymes in diverse and abundant microorganisms. Nat Microbiol 9, 1979–1992 (2024). https://doi.org/10.1038/s41564-024-01715-9.  Creative Commons Attribution 4.0 International License.  http://creativecommons.org/licenses/by/4.0/.

The ‘methylation’ pathway in some higher plants with the methionine (Met) S-methyltransferase (MMT) and bacteria containing MmtN or another methyltransferase (BurB) (left); the ‘transamination’ pathway in algae, bacteria, and corals with DSYB/DsyB, DsyGD/DsyG, DSYE and/or TpMMT (middle); and the ‘decarboxylation’ pathway in Crypthecodinium cohnii (right). The pathways are named after their first reaction step (in larger font). AdoMet, S-adenosylmethionine; AdoHcy, S-adenosylhomocysteine; NADP, nicotine adenine dinucleotide phosphate; MAT, methionine aminotransferase; MR, MTOB reductase; MSM, MTHB S-methyltransferase; DDC, DMSHB decarboxylase; SMM, S-methylmethionine; MTOB, 4-methylthio-2-oxybutyrate; MTHB, 4-methylthio-2-hydroxybutyrate; DMSHB, 4-dimethylsulfonio-2-hydroxybutyrate; MTPA, 3-methylthiopropylamine; MMPA, 3-methylmercaptopropionate. 

DMSP is not a minor microbial product.  Petagrams (10¹⁵) are made in surface water and more in other ocean regions.  Intracellular DMSP (often at millimolar concentrations) is an osmolyte that helps cells cope with the osmotic stress of living in saltwater. It also provides marine organisms with carbon and sulfur. DMS is released into the atmosphere and cycles diurnally, giving the oceans their characteristic smell.  It and DMSP also act as signaling molecules in fish and whales. 

These last two chapters clearly show that humans have been engaged in large-scale geoengineering of the planet and its biosphere. We must develop new methods and technologies to lower the Earth's temperature.  One might be the large-scale production of salt aerosols over the oceans, which would reflect incident sunlight and increase cloud formation.

This chapter shows how our climate fate will depend on our individual and collective choices as societies. 

Summary 

(Summary written by Claude, Anthropic)

Human activities—primarily the combustion of fossil fuels beginning with the Industrial Revolution around 1760—have injected approximately 1.5 trillion tons of CO₂ into the atmosphere, raising atmospheric CO₂ to levels not seen in at least 3 million years. NASA climate models demonstrate unambiguously that only anthropogenic greenhouse gas forcing reproduces the observed global temperature rise since 1880; natural factors, including changes in solar irradiance, volcanic activity, orbital mechanics, and land use, cannot explain the magnitude or rate of current warming. The characteristic seasonal zig-zag in the CO₂ record reflects active Northern Hemisphere photosynthesis temporarily drawing down atmospheric CO₂ each summer. Methane, the second most important anthropogenic greenhouse gas, now accounts for up to ~20% of observed warming and, along with other greenhouse gases, has risen sharply since 1975. Earth's Energy Imbalance (EEI) has risen from ~+0.6 W/m² in 2000 to ~+1.0 W/m² in 2021 and continues to trend upward, reflecting the growing thermal imbalance driving ongoing warming. A revised 2023 climate sensitivity estimate by Hansen et al. of 4.8°C per CO₂ doubling suggests the IPCC's 1.5°C warming target may no longer be achievable and that staying below 2°C before 2050 will require immediate, dramatic action.

Historical climate fluctuations—including the Little Ice Ages (most severe 1450–1850) and the catastrophic 536 AD cooling—provide important context. The 536 AD event, caused by Icelandic volcanic eruptions (and compounded by additional eruptions in 540 and 547), dropped summer temperatures by 1.5–2.5°C, triggered widespread famines, and preceded the Justinian Plague around 541. The Little Ice Ages involved decreased solar irradiance (Maunder Minimum, ~0.06% decrease), multiple volcanic eruptions (most dramatically Tambora in 1815), and changes in Atlantic Meridional Overturning Circulation (AMOC) dynamics—with initial Arctic ice loss in the late 1300s–1400s cooling and diluting North Atlantic waters and collapsing the AMOC heat transfer. These events illustrate the climate system's sensitivity to relatively small radiative perturbations, but also show that their mechanisms differ fundamentally from anthropogenic forcing—and that future human-caused warming, if unmitigated, would produce far more severe and potentially irreversible consequences. Critically, sufficient anthropogenic CO₂ has now been injected into the atmosphere that the orbital insolation minimum required to trigger the next ice age has been pushed far below any naturally occurring threshold.

Climate projections have evolved through successive IPCC frameworks. The original RCP scenarios (Representative Concentration Pathways) characterized futures by radiative forcing (W/m²); the SSP (Shared Socioeconomic Pathways) framework added socioeconomic and political context. The newest CMIP7 framework replaces these with scenarios named by emission trajectories—from Very Low (consistent with ~1.5°C peak warming) through High (below the retired RCP 8.5 boundary)—and incorporates more current data on clean energy deployment progress. Retiring RCP 8.5 as a plausible worst-case scenario reflects real-world clean energy progress, while it retains legitimate value as an upper boundary condition for impact modeling, analogous to Monte Carlo worst-case scenarios in financial planning.

Climate justice is inseparable from climate science. The United States leads all nations in cumulative per-capita CO₂ emissions since the Industrial Revolution; the wealthiest 20% of the global population collectively account for ~86% of total emissions; and nations with minimal historical contributions—such as Pakistan, which experienced catastrophic flooding in 2022—suffer disproportionate climate impacts. The largest historical emitters bear the greatest moral and practical responsibility for rapid emissions reductions, financing clean energy transitions in developing nations, and providing climate adaptation support.

The sulfur cycle adds biochemical complexity to climate feedbacks. Marine phytoplankton, algae, bacteria, and some plants synthesize DMSP (dimethylsulfoniopropionate) from methionine via three pathways (methylation, transamination, or decarboxylation). At millimolar intracellular concentrations, DMSP serves as an osmolyte in hyperosmotic marine environments and as a source of carbon and sulfur for marine organisms. DMSP lyase enzymes (Ddd) cleave DMSP to release DMS (dimethylsulfide), which is released into the atmosphere in petagrams annually, where it oxidizes to sulfate aerosols that reflect solar radiation and promote cloud formation, locally cooling the climate and transferring sulfur to terrestrial ecosystems. This creates an important climate feedback: reductions in anthropogenic SO₂ emissions required by clean air regulations paradoxically accelerate warming by reducing the reflective and cloud-seeding aerosol load—a phenomenon already observed following maritime shipping sulfur restrictions implemented around 2015, which contributed to the recent sharp rise in EEI.

Taken together, this chapter underscores that human activities have become the dominant force shaping Earth's climate, and that individual, institutional, and collective choices about energy use, emissions, and policy will determine which climate scenario future generations inherit.