If Earth was warmer millions of years ago, before humans existed, how do we know current warming is caused by humans and not just a natural cycle?

Natural climate changes do happen—Earth has warmed and cooled before due to changes in solar output, volcanic activity, and orbital cycles. Scientists study those cycles using the same proxy data (ice cores, tree rings) and have models that show how fast climate changed naturally in the past. Natural cycles cause warming of a few tenths of a degree over centuries. The current warming—1.1 degrees in 120 years, and most of it in the last 40 years—is much faster than natural cycles. Also, computer models show that natural factors alone (solar cycles, volcanoes, orbital changes) cannot explain the observed warming without human greenhouse-gas emissions added. When scientists include human CO₂ in their models, the results match observations perfectly. So yes, climate changes naturally, but the speed and timing of current warming point to human cause, not a natural cycle.

Why does temperature wobble up and down year to year if CO₂ is steadily rising?

Year-to-year temperature variations are caused by other factors on top of the long-term warming trend. El Niño and La Niña are ocean-atmosphere patterns that warm or cool the planet for 1–3 years. Volcanic eruptions cool the atmosphere by injecting ash that reflects sunlight. Solar output varies slightly over an 11-year cycle. Ocean currents shift. All these factors create noise in the temperature record—the ups and downs you see on the graph. The CO₂ trend is steady because we keep burning fossil fuels at increasing rates. But temperature responds to everything happening in the climate system at once. Think of it like a boat: the ocean (climate system) has waves (short-term variations) on top of a rising tide (long-term warming from CO₂). The waves don't erase the tide; they just make the path bumpy. When reading temperature graphs, ignore the wiggles and look for the overall direction.

Could the warming be explained by cities growing and getting hotter, without the whole planet actually warming?

Scientists thought about this and tested it. In the 1980s, some people suggested that weather stations in cities record higher temperatures because cities absorb and re-radiate heat differently than the countryside (the urban heat island effect). If that were the reason for global warming, the warming signal should be strongest at cities and weak in rural areas. When scientists analyzed temperature data from rural weather stations, remote islands, and satellites orbiting above cities, they found warming everywhere—cities, countryside, oceans, and upper atmosphere. The warming is global, not just urban. Also, satellites that don't touch the ground confirm warming of the lower atmosphere. So while cities do tend to be warmer locally, that doesn't explain the global warming trend. The whole planet is warming, including areas with almost no cities.

If we stop burning fossil fuels tomorrow, will CO₂ levels drop right away and temperature go back to normal?

No—it would take a very long time. CO₂ stays in the atmosphere for hundreds of years. Even if humans stopped all fossil-fuel burning instantly, the CO₂ already in the air would continue trapping heat, and temperatures would keep rising for several more decades due to the heat already in the oceans. Eventually, some CO₂ dissolves in the ocean or gets absorbed by soil and plants, but that process is slow. The good news is that if we reduce fossil-fuel use now, we prevent even more CO₂ from accumulating, which means we avoid even more warming in the future. The bad news is that because of CO₂ already in the air, Earth will continue warming for a while no matter what we do today. This is why climate scientists emphasize that we need to act soon—every bit of warming we can prevent now matters, because once CO₂ is in the air, we're locked in to continued warming for centuries.

M6SCI-10.2

Evidence for Rising Global Temperatures

Learn how scientists use temperature and CO2 data graphs to show Earth is warming and why fossil fuels are the cause.

What you'll do in this lesson

A voice-first session with the Crimsora tutor on Evidence for Rising Global Temperatures, then targeted practice and FRQs — with the tutor adapting to where you get stuck.

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What this lesson covers

Over the last 100 years, Earth's average temperature has risen, and atmospheric carbon dioxide has increased. But how do we know this is actually happening? Scientists don't have thermometers that recorded global temperature in 1920, so they use different kinds of evidence—tree rings, ice cores, ocean temperatures, and modern measurements—to piece together the story. In this lesson, you'll learn how to read real climate graphs, understand what they show about warming trends, and see why scientists link that warming to human burning of fossil fuels.

How Scientists Measure Past Temperatures

Scientists cannot directly measure global temperatures from 100 years ago, so they use proxy data—indirect measurements that tell us about past climate. Tree rings tell us about past rainfall and temperature because trees grow more in warm, wet years. Ice cores drilled from glaciers contain tiny air bubbles trapped when the snow fell; by measuring the isotopes in that ice and air, scientists can calculate what the temperature was when it froze. Ocean sediments and coral skeletons also preserve chemical clues about temperature. Modern temperature records began around 1880 when weather stations were set up worldwide. Today, satellites measure Earth's temperature from space. All these sources—combined—paint a picture of how temperature has changed over time. When scientists combine proxy data with modern measurements, they create a continuous temperature record going back hundreds of years.

Reading Temperature Trend Graphs

A temperature graph plots time (in years) on the x-axis and temperature change (usually in degrees Celsius above or below a baseline) on the y-axis. The baseline is often the average temperature from 1901 to 2000. When you read a temperature trend graph, look for the overall pattern, not just one year's data. A jagged line with ups and downs is normal—some years are hotter or cooler due to short-term weather variations. What matters is the long-term trend: does the line generally go up, down, or stay flat? To identify the trend, imagine a smooth line running through the data. Over the past 120 years, global temperature has risen by about 1.1 to 1.2 degrees Celsius. The warming has accelerated in recent decades; the last 40 years are much warmer than the 100 years before that. This accelerating warming is key evidence that something has changed in Earth's climate system.

Atmospheric CO₂ Levels and the Carbon Cycle

Carbon dioxide is a greenhouse gas—it traps heat in Earth's atmosphere by preventing infrared radiation from escaping to space. The carbon cycle naturally moves carbon between the atmosphere, oceans, rocks, soil, and living things. Before the Industrial Revolution (around 1750), CO₂ levels stayed relatively stable around 280 ppm (parts per million) because natural processes balanced how much carbon entered and left the atmosphere. When humans began burning fossil fuels—coal, oil, and natural gas—we added huge amounts of carbon that had been locked underground for millions of years. Today's atmospheric CO₂ is over 420 ppm and rising. Graphs show that atmospheric CO₂ and temperature have risen together over the past 150 years. This correlation (they move together) is strong evidence that increased CO₂ is driving the warming. The amount of extra CO₂ in the air since 1750 comes directly from human fossil-fuel burning; scientists can tell because the isotope ratios of the carbon match plant material, not from natural volcanic or ocean processes.

Connecting Fossil-Fuel Use to Rising CO₂

Fossil fuels form when dead organisms are buried underground and compressed over millions of years. Coal, oil, and natural gas are energy-dense and cheap, so humans have burned them to power factories, transportation, electricity generation, and heating. Every time we burn fossil fuel, the carbon in it combines with oxygen to make CO₂, which enters the atmosphere. Global fossil-fuel consumption has grown exponentially since 1950. Graphs of CO₂ levels show a sharp increase starting around 1950—exactly when industrial nations ramped up energy use. The graphs also show seasonal wobbles (CO₂ drops slightly in the Northern Hemisphere spring and summer when plants photosynthesize heavily, then rises again in fall and winter), but the overall trend is unmistakably upward. This pattern—rising fossil-fuel use, rising CO₂ emissions, rising atmospheric CO₂, rising global temperature—forms a chain of evidence that connects human activity to climate change.

What the Data Tells Us and What It Doesn't

Temperature and CO₂ graphs show correlation—the two variables move together—but correlation is not the same as causation. However, scientists have multiple lines of evidence beyond the graphs. We understand the greenhouse effect from physics: CO₂ molecules absorb infrared radiation in the lab. We know fossil fuels are the source of the extra CO₂ by analyzing carbon isotopes. We can model Earth's climate and show that natural factors alone (solar cycles, volcanic activity, orbital changes) cannot explain the observed warming without including human CO₂ emissions. Computer models that include human greenhouse-gas emissions match the observed temperature trend; models without them do not. The graphs themselves show that temperature didn't rise steadily—it leveled off or even dropped in some decades (1940–1970)—while population and fossil-fuel use kept increasing. This variation tells us that climate is complicated and natural factors still matter. But the long-term trend is undeniable: as CO₂ from human activities has accumulated, Earth has warmed.

Key terms

Proxy data.: Indirect measurements of past climate, such as tree rings, ice cores, and ocean sediments, used because direct thermometer records don't exist for centuries ago.
Atmospheric CO₂.: Carbon dioxide gas in the air, measured in ppm (parts per million); a greenhouse gas that traps heat in Earth's atmosphere.
Trend.: The overall direction of change in a dataset, found by looking past short-term ups and downs to see the long-term pattern.
Correlation.: When two variables move together or change in a related way; two things being correlated does not prove one causes the other.
Fossil fuels.: Coal, oil, and natural gas—non-renewable energy sources formed from dead organisms buried underground millions of years ago.
Greenhouse effect.: The process by which certain gases (like CO₂) in the atmosphere absorb infrared radiation and trap heat, warming Earth's surface.
Ppm (parts per million).: A unit of measurement showing the number of molecules of one substance per million molecules of air; used to measure atmospheric CO₂ concentration.
Baseline.: A reference level, often the average value over a past period, used to compare and calculate change in measurements like temperature.

Worked example

A graph shows atmospheric CO₂ rising from 310 ppm in 1960 to 415 ppm in 2020. Global fossil-fuel use increased by 300 percent in the same period. Another graph shows global average temperature rising from −0.1°C below the 1951–1980 baseline in 1960 to about +0.9°C above it in 2020. Describe what these graphs show together and explain what they suggest about human activity and climate.

Start by reading each graph separately. The CO₂ graph shows a clear upward trend from 1960 to 2020, with no major dips or reversals—CO₂ only goes up. The fossil-fuel graph shows it increased 300 percent in the same time, matching the direction of CO₂ change. The temperature graph also trends upward from 1960 to 2020, but notice it's not a smooth line—some years dip and some spike. However, overall, temperatures in 2020 are about 1 degree Celsius warmer than in 1960.

Now look at all three together. All three variables—fossil fuels, CO₂, and temperature—increased over the same 60-year period. The graphs show correlation: the variables move in the same direction. Why is this evidence? It shows that when humans increased fossil-fuel burning (and therefore CO₂ emissions), atmospheric CO₂ rose, and then global temperature rose. This pattern suggests a link. The reason scientists accept this as evidence is that they also understand why CO₂ causes warming: CO₂ is a greenhouse gas that traps infrared radiation, a property confirmed by laboratory experiments and physics. Additionally, scientists know the extra CO₂ comes from fossil fuels by studying carbon isotopes—the isotope ratios in atmospheric CO₂ match plant material (which is what fossil fuels are), not volcanic or ocean sources. So the graphs show a real correlation supported by other evidence. Your answer should say: (1) what each graph shows, (2) that all three trends move together (correlation), and (3) what that suggests about the relationship between human fossil-fuel use and climate warming.

Practice questions

A student looks at two graphs: one showing atmospheric CO₂ from 1900 to 2020, and one showing global average temperature from 1900 to 2020. She notices that CO₂ rose steadily from 1900 onward, but temperature actually fell slightly from 1940 to 1970, even though CO₂ kept rising. She concludes, 'CO₂ cannot be causing global warming because temperatures went down while CO₂ went up.' What is wrong with her reasoning?

Answer: She is confusing short-term variation with long-term trend, and she is ignoring other factors that affect temperature year-to-year.

This is a common mistake. Yes, temperature dipped from 1940–1970 while CO₂ rose—that happened because industrial pollution (sulfate aerosols) in the atmosphere reflected sunlight and cooled the planet, a temporary effect. Natural factors like solar cycles and volcanic eruptions also cause year-to-year and decade-to-decade wiggles in temperature. But when you zoom out and look at the overall trend from 1900 to 2020, temperature has risen. Scientists expect short-term dips and don't expect a perfectly smooth line; they look at the long-term pattern. If you only look at one decade, you'll miss the bigger story. Her graph-reading skill needs work: when you are asked about a trend, always look at the general shape over the full time period, not at single dips or spikes.

Atmospheric CO₂ was about 280 ppm before 1750 and is now over 420 ppm. Explain why scientists know this extra CO₂ comes from burning fossil fuels, not from volcanoes or ocean warming.

Answer: Scientists measure the ratio of carbon-12 to carbon-13 isotopes in atmospheric CO₂ and in different sources. The isotope ratio of atmospheric CO₂ today matches that of fossil fuels and plants, not volcanic gases or ocean-released CO₂.

This answer shows understanding that correlation (CO₂ and temperature both rising) is not enough on its own—you need to know the source of the extra CO₂. Scientists can trace the origin using isotope chemistry. Fossil fuels are ancient plant and animal material, so they have a specific isotope fingerprint. When we burn coal or oil, that fingerprint ends up in the air. Volcanic CO₂ and CO₂ released by ocean warming have different isotope signatures, so they would look different. The atmosphere's isotope ratio matches fossil fuels, proving humans are the source. This is detective work using chemistry, not just looking at a graph. Understanding it shows you grasp why graphs alone, while suggestive, need supporting evidence.

Write a paragraph explaining what evidence scientists have that (1) Earth is warming and (2) that warming is linked to human fossil-fuel use. Use at least two sources of data.

Answer: Scientists know Earth is warming because multiple types of data show rising temperature over the past 120 years. Tree rings and ice cores show past temperatures, proving it was cooler before 1900. Modern thermometer records from weather stations, combined with satellite measurements, show that global average temperature has risen about 1.1 degrees Celsius since 1880, and the warming has accelerated in recent decades. Atmospheric CO₂ has risen from 280 ppm before 1750 to over 420 ppm today, with the steepest increase happening after 1950 when fossil-fuel use exploded. Temperature and CO₂ graphs show a strong correlation—both increased together, especially in the last 60 years. Scientists also know the extra CO₂ comes from human fossil-fuel burning because the isotope ratios of carbon in the air match the isotope ratios of coal, oil, and gas, not of natural sources like volcanoes. Global fossil-fuel consumption has grown exponentially, adding billions of tons of CO₂ each year. All these lines of evidence together—proxy data showing past temperatures, modern data showing current warming, CO₂ measurements showing the gas comes from fossil fuels, and the correlation between all three—convince scientists that humans are warming the planet by burning fossil fuels.

A strong answer does three things: it names at least two data sources (tree rings or ice cores for past temperature, modern thermometer or satellite data for current temperature, CO₂ measurements, fossil-fuel consumption data), it describes what each shows, and it connects them into a logical chain. It shows that the student understands we don't rely on one graph alone but on multiple types of evidence that all point the same direction. The answer also demonstrates that the student knows isotope analysis is how we know the CO₂ comes from fossil fuels, not just that the two graphs go up together. This is the kind of reasoning your teacher is looking for.

FAQ

If Earth was warmer millions of years ago, before humans existed, how do we know current warming is caused by humans and not just a natural cycle?: Natural climate changes do happen—Earth has warmed and cooled before due to changes in solar output, volcanic activity, and orbital cycles. Scientists study those cycles using the same proxy data (ice cores, tree rings) and have models that show how fast climate changed naturally in the past. Natural cycles cause warming of a few tenths of a degree over centuries. The current warming—1.1 degrees in 120 years, and most of it in the last 40 years—is much faster than natural cycles. Also, computer models show that natural factors alone (solar cycles, volcanoes, orbital changes) cannot explain the observed warming without human greenhouse-gas emissions added. When scientists include human CO₂ in their models, the results match observations perfectly. So yes, climate changes naturally, but the speed and timing of current warming point to human cause, not a natural cycle.
Why does temperature wobble up and down year to year if CO₂ is steadily rising?: Year-to-year temperature variations are caused by other factors on top of the long-term warming trend. El Niño and La Niña are ocean-atmosphere patterns that warm or cool the planet for 1–3 years. Volcanic eruptions cool the atmosphere by injecting ash that reflects sunlight. Solar output varies slightly over an 11-year cycle. Ocean currents shift. All these factors create noise in the temperature record—the ups and downs you see on the graph. The CO₂ trend is steady because we keep burning fossil fuels at increasing rates. But temperature responds to everything happening in the climate system at once. Think of it like a boat: the ocean (climate system) has waves (short-term variations) on top of a rising tide (long-term warming from CO₂). The waves don't erase the tide; they just make the path bumpy. When reading temperature graphs, ignore the wiggles and look for the overall direction.
Could the warming be explained by cities growing and getting hotter, without the whole planet actually warming?: Scientists thought about this and tested it. In the 1980s, some people suggested that weather stations in cities record higher temperatures because cities absorb and re-radiate heat differently than the countryside (the urban heat island effect). If that were the reason for global warming, the warming signal should be strongest at cities and weak in rural areas. When scientists analyzed temperature data from rural weather stations, remote islands, and satellites orbiting above cities, they found warming everywhere—cities, countryside, oceans, and upper atmosphere. The warming is global, not just urban. Also, satellites that don't touch the ground confirm warming of the lower atmosphere. So while cities do tend to be warmer locally, that doesn't explain the global warming trend. The whole planet is warming, including areas with almost no cities.
If we stop burning fossil fuels tomorrow, will CO₂ levels drop right away and temperature go back to normal?: No—it would take a very long time. CO₂ stays in the atmosphere for hundreds of years. Even if humans stopped all fossil-fuel burning instantly, the CO₂ already in the air would continue trapping heat, and temperatures would keep rising for several more decades due to the heat already in the oceans. Eventually, some CO₂ dissolves in the ocean or gets absorbed by soil and plants, but that process is slow. The good news is that if we reduce fossil-fuel use now, we prevent even more CO₂ from accumulating, which means we avoid even more warming in the future. The bad news is that because of CO₂ already in the air, Earth will continue warming for a while no matter what we do today. This is why climate scientists emphasize that we need to act soon—every bit of warming we can prevent now matters, because once CO₂ is in the air, we're locked in to continued warming for centuries.

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