Climate Science: a complete guide to global warming
Welcome to my complete guide to climate science. For decades, the media cast doubt on climate science. By now, more than enough theoretical and observational confidence among scientific research has been collected to take action to address climate change.
In this post, you’ll find the following sections:
The science of climate change: How and why the planet is getting hotter
How we know climate change is real
How we know climate change is caused by humans
What is the difference between weather and climate?
Scroll down to learn more.
The science of climate change: How and why the planet is getting hotter
Climate change seems simple: the more greenhouse gases there are in the atmosphere, the more our atmosphere will heat. However, this is level of detail doesn’t capture some critical information.
Using this explanation, people may overestimate how much plant growth can absorb the CO2 produced by the fossil fuels we burn. Or they may not realize that water vapor is even more potent than CO2 for heating the atmosphere.
Here are a few more details to help a layperson understand how climate change really works.
Energy Balance
Worldwide, about 340 W/m2 of energy radiated by the sun reaches the earth.
Here’s what happens to that energy:
30% is reflected back into space by clouds, ice, and other surfaces. This is known as the albedo effect. it also means that the earth will have to absorb more of sunlight’s radiation as global ice and snow melt.
Based on the work of John Fourier in 1820, we now know the earth radiates the remaining 70% back into space as infrared radiation to maintain energy balance. Here’s a brief overview of some of the early findings on climate change in the 19th century.
The theory of blackbody radiation helps explain how this works. In an ideal physical model, a pure blackbody that absorbs all of the light it receives will have to re-emit the same amount of light it absorbs based on the laws of thermodynamics. As the temperature of a body increases, the amount of radiation it re-emits also increases.
The rule of energy balance states that the energy of the sunlight that reaches the Earth must be equal to the energy it re-emits back into space. This determines the temperature of the climate.
RADIATIVE FORCING
Each different greenhouse gas also has a different global warming potential, atmospheric lifetime, level of increase, contributing to a different fraction of total radiative forcing.
The atmospheric lifetime of a greenhouse gas is how long it accumulates in the atmosphere.
The global warming potential reveals how much warming the gas contributes compared to CO2 (used as the baseline) over a 100-year period.
The level of increase of a greenhouse gas is how much its quantity has increased in the atmosphere since pre-industrial times.
Fraction of total radiative forcing is how much it contributes to the rise in the earth’s temperature.
Here is the breakdown of how much impact the most significant greenhouse gases have had on our planet in terms of their radiative forcing:
Carbon dioxide has an atmospheric lifetime of 500 years, a Global Warming Potential (GWP) of 1, with an increase of 135ppm since the pre-industrial period, causing 56% of radiative forcing (added heat to our atmosphere).
Methane has an atmospheric lifetime of 28 years, a Global Warming Potential (GWP) of 28, with an increase of 1.1ppm since the pre-industrial period, causing 15% of radiative forcing.
Nitrous Oxide has an atmospheric lifetime of 121 years, a Global Warming Potential (GWP) of 265, with an increase of 75ppb since the pre-industrial period, causing 5% of radiative forcing.
Halocarbons (HFCs, etc.) have an atmospheric lifetime that varies years to millennia, a Global Warming Potential (GWP) of 100s-1000s, with an increase of a few ppb since the pre-industrial period, causing 11% of radiative forcing.
Ozone has an atmospheric lifetime that varies weeks to months, with an increase of tens of ppb since the pre-industrial period, causing 12% of radiative forcing.
Unlike greenhouse gases, aerosols like sulphur, smog and mineral dust from agriculture actually block sunlight from reaching the earth. Therefore, they have a negative radiative forcing impact on the earth. Aerosols complicate the picture of how much greenhouse gases warm the planet, because they offset part of the heating greenhouse gases cause.
While water vapor is one of the most impactful greenhouse gases, humans contribute little to its change. It is mostly determined by atmospheric temperature, since water and snow evaporate at higher temperatures. Therefore, as the planet heats up, the increase in water vapor will contribute more and more to the rise in temperature. This is known as the water vapor feedback.
Overall, the proportion of change to the radiative forcing of the planet caused by different anthropogenic sources (GHGs and aerosols) is as follows:
CO2 and other GHGs = 3 w/m2
Aerosols = -0.75 w/m2
Total human contribution = 2.25 w/m2 increase in the past 250 years
KEELING CURVE
The first person to begin to comprehensively monitor the amount of CO2 in the atmosphere was Charles D. Keeling, who initiated the measurements in 1957. The measurements show a sharp upward trend, increasing from under 320 ppm in 1960 to about 415 pm by 2020.
We know that the increase in CO2 largely stems from burning fossil fuels. We know this because CO2 molecules from different sources have different numbers of isotopes. Volcanic CO2 differs in its number of isotopes at the molecular level from CO2 emitted from burning fossil fuels.
The increases in CO2 began around the same time as widespread adoption of coal and other fossil fuels as heating and fuel sources. Analyzing the isotopic make-up of the CO2 of air trapped in ice cores reveals what the atmosphere was like before the industrial revolution. It contained roughly 280ppm CO2 in the late 18th century.
On average, the increase in atmospheric CO2 averages 44% of the amount released into the atmosphere in a given year (Global Carbon Project, 2020). This fixed average helps us link the change in greenhouse gases in the atmosphere directly to human activity. So, where does the rest go?
~25% is absorbed by the ocean. This leads to ocean acidification.
~25% is absorbed by new plant growth.
In total, the Keeling Curve and analysis of ice cores reveals that the amount of CO2 in the atmosphere between the late 18th c. and today has increased by about 45% or 135 ppm.
So that covers the main points of why and how the planet is increasing in temperature.
How we know climate change is real
The United Nations Intergovernmental Panel on Climate Change (IPCC) has reported the climate has warmed since the start of the 20th century with an unequivocal (beyond doubt) confidence level.
The IPCC reported that warming was unequivocal in its 2007 IPCC 4th Assessment report (AR4).
This finding was updated in the IPCC’s 5th Assessment report (AR5) published in 2013, when it asserted with a 95% confidence level that the climate had warmed about 0.8C since the period 1850-1900.
The IPCC’s 2021 Sixth Assessment Report, Working Group I (AR6 WGI) updated the increase to about 1.1C.
It drew this conclusion based on numerous independent data sets and statistics that have been thoroughly reviewed for errors or factors that would make the data unreliable. Given the amount of data collected independently and the consistency across the measurements, there is no chance the warming trend could prove false in every case.
Here are the different types of measurements scientists use to verify the planet’s warming trend.
OBSERVATIONAL RECORD OF CLIMATE CHANGE
Scientific observational history covers 150 years. It includes enough of the planet to measure climate over those periods. To determine these trends, scientists use independently verified sources to confirm the surface temperature warming trends. Additional metrics that would correlate to rising surface temperatures such as sea ice extent loss and sea level rise, are used to strengthen the evidence for warming.
The main trends from this record of measurements show the following:
Global average surface temperature rose from ~0C in 1860 to 1.25+C in 2020. This shows a warming of 1.1C on average when you compare the surface temperature averages for the periods 1850-1900 and 2009-2018. By 2021, the warmest years on average since the mid 19th century were: 2016, 2020, 2019, 2017, and 2015.
Global satellite temperature anomalies rose from about -0.3C in 1980 to +0.4 in 2020.
Arctic sea ice extent dropped from about 7.5 millions of km2 in 1980 to 4 millions of km2 in 2020.
Cumulative glacier ice loss dropped from about 0 tonnes/m2 in 1970 to -28 tonnes/m2 in 2020.
Global ocean heat content rose from about -8 10^22 Joules in 1960 to 23 0^22 Joules in 2020—measured for the top 2km/1.25mi of the ocean.
The average global sea level rose steadily (a linear pattern) about 90 mm by 2020 since 1995. The factors leading to sea level rise include the melting of grounded ice, and the thermal expansion of water (water molecules expand with temperature increases). Each factor has had a nearly equal impact on sea level rise.
(Source: Independent verified sources incl. NASA, NOAA, and UK Hadley Center - compiled via GARP SCR Pearson Exam Prep book, 2022)
The distribution across the planet is not even, either.
Land warmed more than the ocean.
The northern hemisphere (home to 85% of the world’s population) warmed more than the tropics or the southern hemisphere.
Here are some additional trends to expect in the future.
Oceans absorbed about 93% of heat trapped by greenhouse gases, leading to marine heatwaves, ocean acidification, and reduced oxygen levels at least to the end of the century.
The water cycle is intensifying, which can bring more rainfall and flooding in some regions and more drought in others.
Rainfall is likely to increase in high latitudes, but decrease in the subtropics. Monsoon precipitation changes will occur, but vary across regions.
Coastal areas will experience dramatic changes: ongoing sea level rise throughout the 21st century and beyond, coastal flooding, and coastal erosion.
An increase in extreme sea level events with a frequency of 1/100 years could increase to 1/1 year by 2100.
The Arctic will see more permafrost thawing, snow cover loss, glacier and ice melting, and summer Arctic sea ice loss.
Cities may experience more extreme heat from the urban heat island effect and flooding due to the coastal area trends.
GEOLOGICAL RECORD OF CLIMATE CHANGE
Scientists don’t only use observational records for understanding climate change. They use a variety of methods to analyze longer periods of time to develop the geological record:
Corals: Coral skeletons contain ocean climate data for millions of years.
Ice cores: The chemical composition of ice and its bonded carbon molecules supports climate estimates for the past million years.
Ocean sediment cores: The mud at the bottom of the ocean can be assessed for climate information spanning millions of years.
Speleothems: Cave structures supply information that provides estimates for nearby regional climate for hundreds of thousands of years.
Tree rings: Reveal changes in the climate for up to a thousand years in the past.
Here are the trends in the long-term historical record based on these measurement methods:
50 million years ago, the planet was much hotter than today with no permanent ice. It has since been cooling.
410,000 years ago, the earth started to cycle between cold and hot periods (hot house earth and ice ages). We are living in an in-between phase following the end of an ice age about 10,000 years ago; it dropped to its coldest temperatures about 20,000 years ago. The average temperature difference between an ice age and an interglacial period is about 6C.
Since the end of the last ice age, the geological period has been labeled the Holocene. Temperatures gradually rose until about 7,000 years ago and then declined until about 200-300 years ago (a mini-ice age). Since then, the earth has warmed, especially with the rapid warming attributed to anthropogenic climate change in the past century.
The current average global temperature is about 1.1C, comparable to the peak temps of the mid-Holocene about 7,000 years ago. The rate of warming is about 16 times faster than than the average rate of warming since the last ice age (about 6C per 100,000 years = about 0.06C per century).
What does the long-term geological record tell us?
Human civilization as we know it started to develop settlements around 14,000 years ago, and modern urban centers as we know them didn’t arise until the industrial revolution in the mid 19th century.
We may soon leave climatic temperatures ranges that supported the development of modern human life.
How we know climate change is caused by humans
Scientists have found that humans have caused climate change, primarily from burning greenhouse gases. Here are the other possibilities they ruled out.
A wide range of earth processes impact our climate.
Techtonic processes: As the plates of the earth shift, different continents can approach the poles and cool down, creating ice sheets which reflect some of the sunlight energy back into space. Alternatively, heating and ice melting could occur if the plates shifted away from the poles. However, this process is extremely slow—too slow to attribute modern climate change to this process.
Solar radiation levels: If the sun had increased its radiation over the past two decades, that could explain climate change. Satellite measurements of the sun’s output since the 1970s do not support this possible explanation for climate change, which show little change.
Variations in Earth’s orbit: If the earth’s orbit moved closer to the sun, it would heat up from the increase in sunlight absorption. While the orbit’s shape, its tilt, and its and the date of the closest position to the sun do vary, the timeframe of change spans tens of thousands of years, so these theories don’t support the rapid increase in temperature from modern climate change, which only spans over two centuries.
“Unforced” variability: The earth exhibits natural changes and cycles that aren’t caused by forcings related to the earth’s energy balance. Examples include the El Niño/La Niña cycles. However, scientists don’t consider this a possible cause of modern climate change for three reasons:
There are no theories or observations to back this up for the time period in question.
Past observations for a millennia don’t exhibit unforced variability at the rate and intensity seen with modern climate change.
Computer simulations show no evidence that unforced variability could contribute to modern climate change.
Greenhouse gases: There is an overwhelming amount of evidence to support human caused greenhouse gases as the cause for modern climate change.
The physical laws of the planet provide a theory outlining how greenhouse gases warm the atmosphere, based on its energy balance.
We have measurements of the amounts of CO2 added to the atmosphere by humans.
The period of warming correlates directly with the period of increased use of fossil fuels as the main sources of heating and fuel.
The geologic record provides evidence that atmospheric CO2 concentration increases have correlated to increases in global temperature in the past.
For these reasons, scientists have concluded that humans have caused climate change.
In 2013, the UNIPCC made the following assertion:
“It is extremely likely that more than half of the observed increase in the increase in the global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together.”
The report qualifies the attribution of climate change in the following ways:
“More than half” suggests that most, but not 100% of the temperature increase can be attributed to humans.
The time period 1951 to 2010 is the period scientists had enough observational evidence to support the claim. This doesn’t mean that it could have been happening longer or earlier outside of this period.
“Extremely likely” has a specific confidence interval of 95% according to IPCC terminology. In other words, a very low 1 in 20 chance existed for mainstream scientific views to be incorrect in 2013, and the confidence interval has increased since.
What is the difference between weather and climate?
It’s important to know the difference between the weather and climate. They are not the same. If you confuse them in winter, you might assume that the climate is cooling, when it’s anything but.
That way when people highlight a given day or season for its notably cold temperatures to deny climate change, you can correct their mistake. They are referring to cold weather patterns, whereas climate change pattern of average temperatures is unequivocally on the rise.
The main difference between weather and climate is the scale of time covered in the analysis.
When you check the weather on your phone, you’re getting a snapshot of the day-to-day changes in the atmosphere. This includes the rain, temperature, wind, and humidity in your local area. Weather tells you about the short-term patterns of change, but it doesn’t reveal long-term trends.
Climate change is the study of long-term trends of atmospheric change across the entire globe. Given the constant variability over each day and year, climate change science looks at statistics across a minimum of a thirty year period.
Climate change science can use these thirty year intervals to compare with other periods. For instance, scientists may compare measurements such as average temperates in the turn of the 20th century (1890-1920) with those of the start of the 21st century (1990-2020).
Sometimes the phrase global warming is used to refer to climate change. “Warming” only describes one aspect to climate change: rising temperatures. It’s true that climate refers to the statistics for average temperature changes over long-periods of time, but temperature is not the only metric used by climate scientists.
Here are the key metrics scientists use to monitor climate changes:
Land surface temperature
Ocean temperature
Precipitation
Humidity
Cloudiness
Visibility
Wind
Sea level
Next time you check the weather, you might also want to check in on climate metrics like the average global warming since industrialization, which is currently about 1.2 degrees Celsius.