Data Dive 3.1

Did global warming stop in 1998?

One temperature record, and a lot of ways to slice it.

You will need paper and a pencil, and a browser for step 3

3.10 / 6Leave

Step 1 of 5 · Your guess

What do you think?

NASA keeps a record of Earth's average temperature going back to 1880. Over the whole record, the planet has warmed about 0.083 degrees Celsius every ten years.

Someone online says warming stopped for fifteen years after 1998. They have a chart with a flat line, and it is real data.

Write it down before you lookHow could a real chart of a warming planet show a flat line? Write down your best explanation.
3.1 · Did global warming stop in 1998?1 / 6Leave

Step 2 of 5 · Look

Every 15-year slice of the record

What every 15-year window says about warming132 windows in one record - the full record trend is +0.083 C/decadetrend, degrees C per decadefirst year of the windowtrend of each 15-year window0.06750.384-0.24901880192019602000

The NASA temperature record, cut into every possible 15-year slice.

How to read it. Each dot is one 15-year slice of the same temperature record. Its height is how fast that slice says Earth was warming, in degrees per decade. Below zero means that slice says cooling.

Find out
  1. Find the lowest dot and the highest dot. Are they from the same record?
  2. How many different answers could someone honestly show you?
3.1 · Did global warming stop in 1998?2 / 6Leave

Step 3 of 5 · Try it

Now you try it, on the same data

This opens the pool, the site's chart tool. The words in bold below are buttons and menus you will see on the screen.

Do these five things

Open the pool on this file: data.sfhsbta.com/thepool?file=window_15_year_trends.csv&mark=dotplot

  1. Under up, pick trend_c_per_decade. This is a dot plot: one dot per 15-year slice. Hover the lowest dot. Which fifteen years, and how fast?
  2. Hover the highest dot. Which fifteen years?
  3. Set keep to to 0. How many slices say the planet was cooling?
  4. Clear keep to. Set keep from to 0.083, the whole record's warming rate. How many slices say faster than that?
  5. What would you want printed on any trend chart before you believed it?
3.1 · Did global warming stop in 1998?3 / 6Leave

Step 4 of 5 · Say it

Your answer, in one sentence

I think warming <did or did not stop> because <what the chart shows> and I checked it by <what you did to the chart>.

The last part matters most. It shows you did not just look at the chart, you tested it.

Here is one exampleI think warming did not stop, because the same record has slices that say cooling and slices that say four times faster. I checked by setting keep to to 0 and counting 29 slices that say cooling, all from one record.
3.1 · Did global warming stop in 1998?4 / 6Leave

Step 5 of 5 · What you found

Check your numbers

-0.249 (1896-1910)
the flattest slice: it says cooling
+0.384 (2011-2025)
the steepest slice: over four times the record
29
slices that say cooling
75
slices that say faster than the whole record
The big idea: the start and end decide the answerWhoever picks where a chart starts and ends can pick its answer. Every slice here is real data, honestly measured. Before you trust a trend, ask why it starts and ends where it does.
One more thing to think aboutSomeone shows you a 15-year chart that agrees with what you already believe. What should you do next?
3.1 · Did global warming stop in 1998?5 / 6Leave

After the dive

The charts you used today

the line

A line follows one thing over time. Read it left to right and watch which way it leans — that lean is the whole story. A dashed stretch is a guess, not something anyone measured.

What it leaves outThe line implies every point between the dots, and nobody measured those. Where you start and stop it decides what the trend looks like.
the dotplot

One dot per thing, stacked up wherever it lands. You can still count every dot, and the piles show where they gather.

What it leaves outAbove a few hundred values the dots run out of room and start to overlap, so the stack stops being an honest count.

library · ladder · dives · the pool · this dive’s worksheet

3.16 / 6Leave