Why Is It So Hot, Already?

Rex Saffer the AstroDoc
16 min readAug 18, 2023


Some say the world will end in fire,
Some say in ice.
Robert Frost

On July 6, 2023, Earth experienced its highest global average temperature ever recorded. And it seems likely, with more than an 80% probability, that 2023 itself will go down as the hottest year in the National Oceanic & Atmospheric Administration (NOAA) data archive, based on a wide variety of sources going back as far as 175 years ago. So, it is fitting that Global Warming and Climate Change are such contemporary hot topics, as it were. These terms frequently are used interchangeably, but they do not mean the same thing.

Global Warming is not a theory, or a myth, or an opinion, or anything other than an established observational fact. The Earth has become measurably, indisputably warmer over the past 175 years. Its average temperature today is the highest since accurate global instrumental measurements became available around 1850.

Temperature Anomaly

Credit: WikiMedia Commons (RCraig09)

The deviation (called the anomaly) is expressed with respect to a standard reference temperature of 13.9 °C (57.0 °F), an average over the period 1950–1980. Like the stock market, there are short–term ups and downs, but the trend is unmistakable. On average, temperatures remained flat through 1900 then increased almost uniformly thereafter. The total variation is a seemingly meager 1.0 °C (1.8 °F), but as we shall see, small variations in temperature can drive large variations in climate.

Climate Change is a related but different matter. It is not the same as Weather, which refers to atmospheric conditions that occur locally and vary over shorter periods of time, from minutes to hours or days. These include winds, clouds, rain, snow, floods, tornados, and the like. Climate refers to long–term measures of regional or global averages of temperature, humidity and rainfall patterns over years to decades.

Weather is when you get six feet of snow overnight, or swelter through five consecutive days of summertime temperatures and humidities in the high nineties (°F). Climate Change is when glaciers melt, sea levels rise, droughts intensify and persist, and the frequency and strength of tropical storms increase to record levels over the long term. Defined as such, Climate Change is also an established observational fact.

If one wishes to discount or deny that Global Warming or Climate Change are real, it is necessary to define what those are and to marshal one’s evidence before crying Wolf. It is insufficient — intellectually and morally dishonest, really — to allege that something is Fake News or a Hoax without doing our homework. What is not clear without deeper investigation is whether Global Warming and Climate Change are causally related. And if so, what induces Global Warming? A major clue comes from the amount of carbon dioxide (CO2) in the Earth’s atmosphere.

The Concentration of Atmospheric CO₂

Direct, accurate measurements of atmospheric CO₂ first became available in 1958. Click here to view an animation showing changes over time. Play it from the start through the 45 second mark (0:45), then pause. This screenshot shows the uninterrupted, monotonic increase from about 315 parts per million (ppm) to 400 ppm in January 2014.

Atmospheric CO₂ Concentration

Credit: National Oceanic & Atmospheric Administration (NOAA)/University of Colorado

The small annual CO₂variations are synchronized with annual temperature variations. When CO₂ is high, temperatures are high. When CO₂ is low, temperatures are low. In statistical language, they are correlated. Long term average trends over the last 70 years are also correlated, as seen in the next illustration.

Temperature Anomaly (Black) on Atmospheric CO₂ (Blue)

Credit: The 2 Degrees Institute

What about measurements from the more distant past? Over millennia, snowfall accumulates and is transformed into continental ice sheets in polar regions, primarily Greenland and Antarctica. On average, these sheets are 3.5 miles thick. Bubbles of air trapped in the ice can be retrieved from drilling cores and analyzed to determine chemical composition. The depth of the recovered bubbles is a measure of their age.

Resume the CO₂ animation and pause again at 1:00:

Atmospheric CO2 Concentration

Credit: National Oceanic & Atmospheric Administration (NOAA)/University of Colorado

Aha. With minor shorter–term variations, historical atmospheric CO2 remained nearly constant from the year 1000 until around 1800. This is the preindustrial CO₂ level, around 280 ppm. After 1800, the rate of increase became moderate, then astonishingly large by 1950, the kind of growth termed exponential. By January 2014 the level had risen to almost 400 ppm. Nine years later, additional data show a further rise to 425 ppm.

What happened around 1800? The Industrial Revolution in the Western world! It ushered in new manufacturing processes and increased automation, particularly in the textile industry. Improved transportation networks (predominately railroads) led to expansion of domestic markets, an enormous increase in wealth, and a rapidly growing population and workforce to produce and consume goods. Later, the electrification of urban and rural areas accelerated the trend. And what powered this Revolution? Energy derived from the burning of coal, then petroleum, then natural gas — fossil fuels. Humans began to pump millions, then billions of tons of CO2 into the atmosphere annually; by 1900, over a billion tons; in 2022, 37 billion tons. This is just from sources related to energy production.

Variations over Geologic Time

Resume the CO₂ animation and play it through to the end to see the behavior over the last 800,000 years. Note that preindustrial atmospheric CO₂ never rises above 300 ppm during that period. Further, the correlation of temperature with atmospheric CO₂ is not a recent thing. These have been in lockstep throughout terrestrial history. Here is the record for the last 400,000 years:

Temperature Anomaly (Black) on Atmospheric CO₂ (Blue)

Credit: The 2 Degrees Institute

However, correlation does not imply causation, which must be established independently. Do rising temperatures promote rising atmospheric CO₂, or vice versa? Does some third factor drive them both? The answer is more complicated than a simple yes or no. Temperature and CO₂ are intimately entwined in a temperature–dependent, closed feedback loop called the carbon cycle.

Running Hot and Cold

The temperature variations in the previous Figure are also a record of ice age glaciation. An ice age is a period of millions to tens of millions of years when global temperatures are systematically low, and large areas of the Earth are covered by glaciers and continental ice sheets. During an ice age, there are recurring, temporary warm periods (interglacials) when glaciers and ice sheets melt and retreat, alternating with cold periods (glacials) when they grow and advance.

Ice age glacial/interglacial cycles come and go due to a recurring interaction between two of Earth’s astronomical motions: 1) The Earth’s annual orbit around the Sun is slightly elliptical, and it receives more energy overall when closer to the Sun. 2) Superimposed on annual variations, the Earth’s rotation axis wobbles slightly, and more Solar energy is absorbed at high latitude regions when the poles are more tilted towards the Sun. Every 100,000 years or so, these factors reinforce each other, and the Earth becomes systematically warmer than average. At other times they oppose, and the Earth becomes cooler.

Ice sheet growth during glacials is a relatively slow process. Winter snowfalls partially melt in summer, and only the net accumulation governs the ice sheet growth rate. Conversely, during interglacials, melting during summer months is offset by a much smaller accumulation during winter. Ice sheets therefore shrink and retreat more rapidly than they grow and advance. This is an oversimplification; there are also important and quite complicated interactions that modulate the average effect of the interacting astronomical cycles: existing ice sheet thickness, global ocean current distributions, prevailing wind patterns, and more.

Northern Hemisphere Ice Sheet Coverage

Credit: National Park Service/NOAA/LandSAT

Our Very Own Ice Age

The most recent ice age, the Quaternary, began about 2.6 million years before the present (BP). At that time, our hominid grand–ancestor Homo Habilis began evolving into Homo Erectus (1.8 million BP), then into Homo Sapiens by 300,000 BP. Waves of human migration out of Africa as early as 1.5 million BP seeded human populations in Europe and Asia.

The Quaternary ice age continues through the present day. Yes, we are still in an ice age, in the relatively warm Holocene interglacial. The most recent warming trend began about 20,000 BP and became fully established about 12,000 BP. The previous Pleistocene interglacial began 120,000 BP, long after modern humans (H. Sapiens Sapiens) and Neanderthals (H. Sapiens Neanderthalis) branched off from our common ancestor H. Erectus. We are not descended from Neanderthals; we are more like cousins on the family tree.

Our two Sapiens subspecies coinhabited the Earth until quite recently. The youngest Neanderthal fossil remains have been carbon dated to 36,000 BP, about the time modern humans developed abstract, symbolic language systems. Eros (erotic love) is at least that old, because the modern human genome in Europeans is contaminated (or enriched?) with 1–2% Neanderthal DNA. In comparison, there is almost none in Africans, consistent with “out of Africa” migration patterns.

During glacials, temperatures decline slowly and steadily, while the onset of interglacials and rising temperatures is more abrupt. This is due in part to the nature of ice sheet advance and retreat cycles described previously. We now turn to the carbon cycle.

Sources and Sinks

A carbon source emits CO₂, while a carbon sink absorbs CO₂. The oceans are simultaneously a large source and a large sink. This is due to the dependence of CO₂ solubility on temperature. Cold water can hold more dissolved CO₂ than warmer water. When ocean temperatures decrease during glacials, more CO₂ goes into solution and is removed from the atmosphere, lowering the atmospheric concentration. During interglacials, ocean temperatures rise and CO₂ is driven out (outgasses) back into the atmosphere. The annual ocean CO₂ absorption and emission budget is between 250 and 350 billion tons. This would be analogous to a checking account where large sums are both deposited and withdrawn. If total deposits are about the same as total withdrawals, the account balance remains relatively constant in the long term.

On land, the respiration (breathing) of living things, their eventual decay and decomposition when they die, and rainwater runoff from limestone formations are carbon sources,. Photosynthesis by microbes and vegetation is a sink. The annual land CO₂ budget is about 450 billion tons. These both dwarf the 37 billion tons of anthropogenic (human–caused) emissions in 2022. Why does this excess have such an outsized effect? Critically, it is that for almost all of terrestrial history, natural processes have been in a delicate dynamical balance, resulting in relatively stable atmospheric CO₂ with a moderate range of variation from 200 to 300 ppm. Anthropogenic CO₂ emissions have stressed the system beyond its ability cope with the additional burden. The oceans have tried to absorb the additional CO₂, but they were already near capacity and have been able to absorb only about 40% of the excess. The remainder builds up unremittingly in the atmosphere.

Not all anthropogenic CO₂ emissions are due to burning fossil fuels. Remarkably, the production of concrete is a significant contributor. Concrete is the most widely used artificial material on Earth. The only resource we consume in larger quantities is water itself. In 2016 it was responsible for about 8% of total global anthropogenic CO₂ production. Changes in land use patterns such as deforestation and agriculture contribute an additional 10–15%.

Global Warming also results in longer, more intense droughts and higher sustained winds that transform forested areas into giant tinderboxes. These feed more frequent, more intense wildfires and produce a measurable increase in CO₂ emission. While wildfires are formally “natural” in some sense, humans are ultimately responsible for global climate and habitat changes that make their frequency, intensity, and consequences more severe.

Historical usage data are relatively straightforward to obtain, and the amounts of CO₂ released by burning various fossil fuels are well known. Fossil CO₂ also has a different chemical signature than other sources and can easily be distinguished from them. The evidence strongly and conservatively indicates that at least 80% of the observed increases over the last 200 years are due to fossil fuel sources.

Cause and Effect vs. Effect and Cause

CO₂ is a well known “greenhouse gas”. During the day, solar radiation in the visible part of the electromagnetic spectrum heats both land and sea. At night, this energy is re–radiated at infrared wavelengths. CO₂ is one of several gases that is a powerful absorber of infrared radiation, forming an effective “blanket” that prevents some of the infrared energy from escaping back into space. The emission rate of methane, a significantly more powerful absorber than CO₂, has also increased dramatically in the past two decades.

The Greenhouse Effect

Credit: Getty/Filo

As the Earth cools during glacials, atmospheric CO₂ is slowly reabsorbed by the oceans, reducing the greenhouse effect and maintaining the rate of cooling and ice sheet growth. Snow and ice are excellent reflectors of solar radiation, so more ice means less energy reaches land and sea and cooling continues. The growth of ice sheets is slow and steady, and the rate of cooling remains relatively constant.

When warming due to periodic astronomical cycles begins again, two independent factors provide positive feedback and accelerate the process. 1) As temperatures rise, glaciers and ice sheets retreat relatively rapidly. Less ice means less energy is reflected, more energy is absorbed by land and sea, and the warming trend is reinforced. 2) Simultaneously, dissolved oceanic CO2 outgasses into the atmosphere, intensifying the greenhouse effect and accelerating the warming trend, which melts more ice, etc., and temperatures rapidly increase until they reach an interglacial equilibrium.

The Long View

Climate Change in the Past

Quaternary interglacials have lasted for 20,000 years, on average. Our Holocene interglacial became fully established 12,000 years ago, and “normally”, whatever that means, we would expect the slide into the next glacial period to be imminent (“any millennium now”, in the wry words of one scientist) or to already have begun. Indeed, it might have, at least until recently, since temperatures had been trending downward for some five thousand years. Some climate models predict a delay of the next glacial by tens of thousands of years, or more.

Temperature Anomaly (Black) on Atmospheric CO₂ (Blue)

Credit: The 2 Degrees Institute

This is a matter of no small concern. Glacial cycles are predominately cold, perhaps as much as 90% glacial and only 10% interglacial. Increased ice sheet coverage during glacials reduces the extent of habitable regions and stresses human populations. A modern day glacial period would be catastrophic, rendering many densely populated areas uninhabitable and drastically reducing the amount of arable land. Sea levels could drop 100 meters or more, closing off shipping channels and transforming existing seaports into useless, landlocked centers of paralyzed distribution dozens or hundreds of miles from the ocean.

Glacial cycles have strongly influenced human population growth and migration in the past, not all in a negative manner. Near the end of the last glacial period, lowered sea levels exposed a vast swath of dry land, the so–called Bering Land Bridge, between Siberia and western Alaska. This enabled migration of Asiatic populations into North America. The massive Laurentide ice sheet covering Canada and the northern U.S. blocked further movement, but subsequent warming removed that barrier and allowed the resumption of migration southward.

Conversely, modern humans may have come perilously close to extinction about 70,000 BP during the depths of the previous glacial period, which experienced some of the coldest temperatures in the whole of the Quaternary ice age. The Sumatran Toba supervolcano eruption, by far the largest of the Quaternary, blasted enormous amounts of globe–circling ash into the atmosphere and initiated a “volcanic winter”. Human populations may have been reduced to a few thousand individuals, concentrated in isolated pockets in tropical regions. Such a catastrophic reduction in numbers is termed a “genetic bottleneck”, because genetic diversity is drastically reduced in highly isolated populations. Humanity obviously survived, but it might have gone the other way.

Climate Change in the Present

As we have defined it here, Climate Change refers to sustained long–term variations in regional or global averages of temperature, humidity and rainfall patterns over years to decades. Here is an expanded list of “Signs of Global Warming” based on data collected by the Union of Concerned Scientists.

1) The area covered by Arctic sea ice in summer has been smaller every year for the past 30 years.
2) The amount of heat absorbed by the upper 0.5 km of the oceans has increased significantly over the past two decades. Warmer ocean waters damage coral reefs, threaten marine ecosystems, and disrupt global fisheries.
3) Air temperatures over oceans, and 4) increasing surface water temperatures both lead to increased evaporation that can feed heavy precipitation events and more powerful hurricanes.
5) Global sea level is rising, threatening coastal populations and freshwater supplies.
6) Humidity is increasing, see 3) and 4) above. Water vapor is also a powerful greenhouse gas, accelerating the warming trend.
7) Glaciers are melting, and snow and ice cover are decreasing, allowing more solar radiation to be absorbed by land and sea and leading to further warming. If all glaciers melt, along with the Greenland and Antarctic ice sheets, sea level could rise as much as 100 meters.
8) Permafrost in Arctic tundra regions is melting, with consequences such as landslides and sinkhole formation. Such an event recently caused a catastrophic collapse of a Russian petroleum waste storage facility, ruinously polluting a nearby river. Vast amounts of carbon, up to 50 billion tons annually, could also be released into the atmosphere.
9) Habitat destruction, pollution, and other factors are causing an ongoing mass extinction of plant and animal species; according to some projections, 20% of all species on Earth will be extinct within the next 25 years.

These are not possibilities — they all are observed to be taking place.

Climate Change in the Distant Future

What is distant? Well, there is short–term distant, 300 to 500 years, and long–term distant, tens to hundreds of thousands of years. In the short–term, much depends on whether carbon emissions can be drastically reduced in time to avoid a so–called “tipping point” of a 2 °C (3.6 °F) rise in average global temperatures. The Earth is a complex, highly interconnected non–linear physical system. Systems like this can remain stable for a long time until some aspect of global behavior reaches a critical limiting value. Climate models, and there is some disagreement about this, suggest that if temperature rises beyond 2 °C by 2030 to 2050, sea ice, glacier, and ice sheet melting would increase to a point where it could not be reversed, even if CO₂ emissions were miraculously cut to zero and carbon sequestration technologies began to remove CO₂ from the atmosphere.

Unfortunately, recent Intergovernmental Panel on Climate Change (IPCC) studies suggest that even if current national pledges to reduce greenhouse-gas emissions are implemented, and that seems increasingly improbable, they are likely to result in at least 3 °C of global warming. The Apocalypse might be upon us no matter what we do, good or bad.

In the very long–term, there is reason to be hopeful, at least from the perspective of the Earth as a closed ecosystem. If the current crisis is due largely to the burning of fossil fuels, and to us this seems inescapable, it cannot go on for much longer.

Fossil fuels are created by anaerobic decomposition of animal and plant remains on land, and of marine organisms, largely algae and plankton, in the absence of oxygen. This happens when geologic processes transport the buried remains to deep crustal layers having high temperatures and pressures. Over the course of tens to hundreds of millions of years, plant remains are transformed into coal, while marine remains are transformed into petroleum and natural gas.

Most fossil fuel reservoirs known today were formed in just two episodes, one about 350 million and the second 70 million years ago, in the Carboniferous and Jurassic/Cretaceous geologic epochs of the Paleozoic era.

Temperature Anomaly (°C)

Credit: NOAA/National Climatic Data Center

The very long, strong interglacials of the Cambrian and Devonian epochs, with no glacial period to speak of between them, were two of the warmest periods in the last 600 million years.

Vegetation diversified and became well established during the Devonian, and the Earth became a worldwide hothouse jungle, with a massive increase in plant biomass. Much of this perished in the Permian glacial and ultimately created a large fraction of the fossil fuel reserves known today. A second, smaller episode took place in the late Cretaceous and early Paleocene epochs.

There may never be another significant fossil fuel producing episode in the time remaining to us on Earth, and it is later than you might think. The Sun continuously becomes hotter and produces more radiation as it ages, and it will be no more than another one billion years before the Earth’s oceans evaporate away completely. Life may not end entirely, but we would not recognize it. By the way, this is one good reason to keep exploring ever more distant regions of the Solar System, and eventually, nearby star systems. If humanity is to survive, it will have to be elsewhere. This might not be as tragic as it sounds. How many of us still live in the places we were born? Migration is the essence of human cultural evolution.

What is most significant is that fossil fuel reserves existing at the beginning of the Industrial Revolution took tens to hundreds of millions of years to form, while we have extracted and burned a large fraction of that supply in just 200 years. Quite likely, not much more is going to be discovered, which is why fossil fuels are nonrenewable. How much is left? Estimates vary, but coal will be gone in 100–200 years, and petroleum and natural gas in 50–100 years. Most studies favor the lower end of these ranges. Very soon, the laws of supply and demand are going to take care of the problem for us, and we no longer will be contributing to rising atmospheric CO₂ — we will not be able to. Of course, we will then have a new problem of colossal proportions; how will we continue to power our energy–greedy industrial societies? Will we have made the transition to a fully renewable energy ecosystem, or will a devastating crash in human populations create another genetic bottleneck or wipe us out altogether, solving the problem that way?

In the long run, it does not matter if we survive. With or without us, the Earth will take care of CO₂ the way it always has and always will. On land and in the sea, living organisms will continue to incorporate carbon into their tissues. Much of this organic carbon will be oxidized back into CO₂ and returned to the atmosphere. But some will decay anaerobically and become locked away in crustal rocks, or become crustal rock in the form of limestone (calcium carbonate, Ca₂O₃). Eventually, excess atmospheric CO₂ will be reabsorbed into the oceans and likely return to preindustrial levels. The Earth will have returned to its natural dynamic equilibrium, though there may be no remaining intelligent life to record the event. The jury is still out deliberating whether legitimately intelligent life currently exists on this planet anyway. Only time — deep time, geologic time — will tell if the world will end in fire or in ice.

All the best,
From Broomall, PA
Friday, August 18 at 7:30 AM,



Rex Saffer the AstroDoc

Retired Physics Professor, Motorcyclist, Bridge Player, Voracious Reader, Philosopher, Essayist, Science/Culture Utility Infielder