Back to the on-screen lesson ·
Instrumental records measure directly but recently; proxies reach far back indirectly; anomalies and trends read them, and their agreement makes the case.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
By the end of this lesson you will be able to compare instrumental and proxy evidence by what each can show, and read anomalies and trends from a record.
You know why adding greenhouse gases shifts Earth's energy balance, and you can find a difference and a rate. This lesson asks how we know the climate is actually changing, and what each kind of evidence can and cannot show.
| Term | What it means |
|---|---|
| Weather | The state of the air at one place and time. |
| Climate | The average weather of a place over decades, usually thirty years. |
| Instrumental record | Measurements made directly with instruments. |
| Proxy | A natural record, such as tree rings or ice cores, that stands in for past conditions. |
| Baseline | The average over a reference period that readings are compared with. |
| Anomaly | A reading minus the baseline: how far above or below normal it is. |
| Trend | The long-term direction of a record, found by looking past year-to-year ups and downs. |
Climate evidence comes in two kinds:
Records are read as anomalies and trends:
$$\text{anomaly} = \text{reading} - \text{baseline}, \qquad \text{rate} = \dfrac{\text{change}}{\text{time}}.$$
The case for change is strong because independent records agree.
Another way: picture
Picture a detective with several witnesses who never met. A thermometer, a tide gauge, a satellite, a tree and a column of ice each saw part of the story. If they all describe the same change, their agreement is far more convincing than any one account alone.
Another way: steps
Weather is what happens today: a cold morning, a storm, a heat wave. Climate is the average of the weather over decades. A single cold winter says little about climate, just as one bad day says little about a school year.
So climate evidence looks for trends over thirty years or more. Scientists compare each year with a baseline, often the average of a thirty-year reference period, and ask whether the anomalies are drifting in one direction.
An anomaly is a reading minus the baseline. If a station's baseline mean is $57$ °F and this year's mean is $58.6$ °F, the anomaly is $1.6$ °F. A reading below the baseline gives a negative anomaly.
Anomalies make places comparable. A mountain station and a desert station have very different temperatures, but both can show an anomaly of one degree, which lets their records be averaged into a regional or global picture.
Thermometer records at weather stations begin in the 1800s in many places, and ships measured the sea surface. NASA and the National Oceanic and Atmospheric Administration combine thousands of these records into a global temperature.
They find that Earth's average surface has warmed by about two degrees Fahrenheit since the late 1800s, and that 2023 was the warmest year in the record. Each of the last several decades has been warmer than the one before.
Instrumental records need care. A station moved to an airport, a new instrument, or a city growing around a station can shift readings for reasons unrelated to climate. Cities are warmer than the countryside around them.
Scientists compare each station with its neighbors to find and correct such shifts. The warming appears just as clearly in rural stations, in ocean measurements and in satellite readings of the air, which rules out cities as its cause.
Since the late 1970s satellites have measured sea ice, sea level, snow cover and the temperature of the air, covering oceans and remote regions that thermometers miss.
The National Snow and Ice Data Center reports that the Arctic's sea ice at its late-summer minimum has shrunk by about thirteen percent per decade since 1979. Satellite altimeters show global sea level rising, and rising faster in recent decades than in the 1990s.
Each year a tree adds a ring of wood, wide in good growing years and narrow in cold or dry ones. Counting inward from the bark dates each ring; its width records that year's growing conditions.
Bristlecone pines in California's White Mountains live for thousands of years, so their rings, matched with those of dead trees, extend the record across millennia. Tree rings are a proxy: they record temperature and rainfall together, so reading them takes careful comparison with instrumental years.
Snow that builds up on Greenland and Antarctica compresses into ice, trapping bubbles of the air of its time. Drilling out a long column of ice and analyzing the bubbles reveals past carbon dioxide levels.
Antarctic cores reach back about $800000$ years. They show that carbon dioxide rose and fell with the ice ages but never came near today's level. The makeup of the ice itself records past temperature, so the cores show carbon dioxide and temperature moving together.
Mud at the bottom of lakes and oceans builds up in layers containing pollen and the shells of tiny organisms. The kinds of pollen reveal past vegetation, and the chemistry of the shells records past ocean temperatures.
Corals, cave formations and boreholes add more records. Each proxy has its own strengths and blind spots, so scientists combine many of them rather than relying on any one.
Tide gauges have measured sea level at some harbors since the 1800s; the gauge at the Battery in New York City began in 1856. Global sea level has risen about eight to nine inches since 1880, according to NOAA.
Seas rise as warming water expands and as glaciers and ice sheets melt into the ocean. Local sea level also depends on whether the land is sinking or rising, so tide gauges at different coasts show different rates.
Checking an answer. An anomaly is negative when the reading is below the baseline. A rate per decade is smaller than the total change whenever more than ten years pass.
Subtracting a baseline is allowed because it removes each place's normal level and leaves only the change, which is the part that can be compared and averaged across places.
Using a proxy is allowed once it has been calibrated: scientists compare it with instrumental readings during the years they overlap, and use that relationship to read the earlier years. Where proxies and instruments overlap, they agree.
Instrumental records are precise but short, so on their own they cannot say whether today's warmth is unusual over thousands of years. Proxies reach back further but are less precise and often record several conditions at once.
Satellites cover the whole planet but only since the 1970s. A single station is long but local. Knowing each record's limits is what lets them be combined into one reliable picture.
Evidence also comes from everyday records. Growing seasons across much of the United States have lengthened as spring frosts come earlier and fall frosts later, according to EPA climate indicators. Lakes freeze later and thaw earlier.
Birds migrate earlier, plants flower sooner, and the ranges of some species have moved north or uphill. None of these alone is proof, but together they point the same way as the thermometers.
Each line of evidence is independent: a tree ring knows nothing of a satellite, and an ice core nothing of a tide gauge. If one were wrong, it would disagree with the rest.
Instead they agree: the air is warming, ice is melting, seas are rising and carbon dioxide is higher than at any time in hundreds of thousands of years. That agreement is why scientific bodies around the world regard the change as established.
The most common slip is to treat one warm or cold year as proof for or against climate change. Another is forgetting that an anomaly below the baseline is negative.
A third is to confuse a proxy with a direct measurement, or to forget how far back each record reaches. A fourth is to give a total change when a rate per decade is asked for.
High in the White Mountains of eastern California grow Great Basin bristlecone pines, some more than four thousand years old. In the dry, cold air, dead trees can lie on the ground for thousands of years without rotting.
Scientists at the University of Arizona's Laboratory of Tree-Ring Research, founded by A. E. Douglass in the 1930s, matched the ring patterns of living and dead bristlecones to build a continuous record reaching back more than eight thousand years. Distinctive sequences of narrow and wide rings let a dead tree's rings be lined up with a living tree's, year by year.
The record has uses beyond climate. It was used to check and correct radiocarbon dating, and ring widths reveal past droughts in the American Southwest, including the severe dry spells of the late 1200s. Climate scientists compare the rings with thermometer readings for the years they overlap, then use them to see how unusual recent conditions are.
At the southern tip of Manhattan, a tide gauge has recorded the level of New York Harbor since 1856, one of the longest records in the country. NOAA publishes its readings, which show sea level there rising roughly a foot since 1900.
Part of that rise comes from global sea level, as oceans warm and ice melts. Part comes from the land around New York slowly sinking, a lingering effect of the last ice age. A tide gauge records the combination, so its trend differs from a gauge in, say, Alaska, where the land is rising.
The record mattered when Hurricane Sandy struck in 2012. Its storm surge rode on a sea about a foot higher than a century before, so it flooded more of the city's subways and tunnels than the same storm would have in 1912. City planners now use the gauge's trend, together with satellite data, to decide how high to build new seawalls.
It is natural to take a very cold winter as evidence that the climate is not warming, or a very hot summer as proof that it is. But climate is the average over decades, and individual years swing above and below the trend because of weather patterns such as El Niño.
The evidence for change is the trend: decades of anomalies drifting upward, confirmed by independent records from thermometers, satellites, tide gauges, trees and ice. A single year, warm or cold, is one point around that trend.
A station's baseline mean is $52$ °F. In 1990 its mean was $52.3$ °F. Find the anomaly.
$52.3 - 52 = 0.3$
Reading minus baseline.
In 2022 the mean was $53.4$ °F. Find the anomaly.
$53.4 - 52 = 1.4$
Reading minus baseline.
Find the change between the two years.
$1.4 - 0.3 = 1.1$
Later minus earlier.
Say what the two years alone cannot show.
$\text{a trend}$
Decades of data are needed.
A region's smoothed anomaly was $0.1$ °F in 1970. Record it.
$0.1$
The earlier value.
It was $1.3$ °F in 2020. Find the change.
$1.3 - 0.1 = 1.2$
Degrees Fahrenheit.
Count the decades.
$\dfrac{2020 - 1970}{10} = 5$
Five decades.
Find the rate.
$\dfrac{1.2}{5} = 0.24$
°F per decade.
Name evidence that could confirm it.
$\text{satellites, oceans, growing seasons}$
Independent records.
A tree cut in $2015$ has its outermost ring from that year. Date ring $1$.
$2015$
The cutting year.
Date ring $2$.
$2014$
One year earlier.
Write the year of ring $n$.
$2016 - n$
Each ring inward is a year earlier.
Date ring $100$.
$2016 - 100 = 1916$
A century of growth.
Ring $100$ is unusually narrow. Say what it suggests.
$\text{a poor growing year in 1916}$
Cold or dry.
Say how to tell cold from dry.
$\text{compare with other proxies and records}$
One ring cannot say which.
Write the anomaly formula.
$\text{reading} - \text{baseline}$
The difference from normal.
Substitute the values.
$59.4 - 60$
Degrees Fahrenheit.
Evaluate the anomaly.
What can the rings of a bristlecone pine in California tell us about the climate?
Complete the worked solution: a tide gauge recorded a mean sea level of $7000$ millimeters above its datum in $1964$ and $7132$ millimeters in $2008$. Find the rise, the average rate in millimeters a year, and the rise over the next thirty years if that rate holds.
Find the rise.
$\text{later} - \text{earlier} =$ r
Millimeters.
Divide by the years between.
$\dfrac{\text{rise}}{\text{years}} =$ k
Millimeters a year.
Project thirty more years.
$30 \times \text{rate} =$ f
If the rate holds.
Say what could make the projection too low.
$\text{the rate is speeding up}$
Satellites show faster rise in recent decades.
Match each term to its meaning.
| measurements made directly with instruments | a natural record that stands in for past conditions | the average over a reference period that readings are compared with | how far a reading is above or below that average | |
|---|---|---|---|---|
| instrumental record | ||||
| proxy | ||||
| baseline | ||||
| anomaly |
A weather station's baseline mean temperature is $48.5$ °F. Its mean was $49.1$ °F in 1995 and $50.3$ °F in 2023. Fill in the anomaly for each year and the change between them, in °F.
| value | |
|---|---|
| anomaly in 1995 (°F) | |
| anomaly in 2023 (°F) | |
| change from 1995 to 2023 (°F) |
A tree was cut in $2010$, and its outermost ring grew that year. Counting that ring as ring $1$ and moving inward, write the year that ring number $n$ grew as a function of $n$.
Answer:
A region's smoothed temperature anomaly was $0.5$ °F in $1990$ and $1.7$ °F in $2020$. At what average rate did it warm, in °F per decade?
Answer: °F per decade
Suppose records from an orchard in Michigan show that in the 1970s the last spring frost came, on average, on day $120$ of the year and the first fall frost on day $270$. In the 2010s the averages were day $111$ and day $276$. By how many days did the growing season lengthen?
Answer: days
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
A tree was cut in $2023$, and its outermost ring grew that year. Counting that ring as ring $1$ and moving inward, write the year that ring number $n$ grew as a function of $n$.
Answer:
You can weigh climate evidence. Explain why one very cold winter does not show that the climate is not warming.
26. Your turn: a station's baseline mean is $60$ °F and this year's mean is $59.4$ °F. What is the anomaly?, step 3
$-0.6$
Below the baseline.