Why Truncated Axes Can Distort Environmental Trends

Current figure

The global average temperature anomaly for 2023 was 1.17°C above the 1951–1980 baseline, according to NASA’s Goddard Institute for Space Studies (GISS) Surface Temperature Analysis (GISTEMP v4). This value represents the combined land and ocean surface temperature departure from the long-term average.

Measurement date

The figure refers to the full calendar year 2023 (January through December). NASA GISS updates its global temperature record annually, typically releasing the previous year’s data in mid-January. The 2023 value was published on 12 January 2024.

Previous figure

The global average temperature anomaly for 2022 was 0.89°C above the 1951–1980 baseline (NASA GISS). The change from 2022 to 2023 was an increase of 0.28°C, or approximately 31.5% relative to the 2022 anomaly. This jump was partly influenced by the transition from La Niña to El Niño conditions in the Pacific Ocean.

Long-term trend

The long-term global temperature trend shows a clear warming of approximately 0.8°C since 1880, with the rate of warming nearly doubling since 1975. The ten warmest years in the instrumental record have all occurred since 2010. The following table presents annual global temperature anomalies (relative to the 1951–1980 baseline) for selected years, based on NASA GISS data:

When plotted on a chart with a truncated y-axis (e.g., starting at 0.5°C rather than 0°C or a pre-industrial baseline), the recent warming appears more dramatic, and the long-term context is lost. A full-axis plot reveals that while 2023 was exceptionally warm, the overall trend since the late 19th century is a steady rise, not a sudden spike.

Data source

The primary data source is the NASA Goddard Institute for Space Studies (GISS) Surface Temperature Analysis (GISTEMP v4). This dataset combines land surface air temperatures from the Global Historical Climatology Network (GHCN) with sea surface temperatures from the Extended Reconstructed Sea Surface Temperature (ERSST v5) dataset. The data are publicly available at https://data.giss.nasa.gov/gistemp/.

Methodology

NASA GISS calculates global temperature anomalies by comparing measured temperatures at weather stations and sea surface temperature readings to a baseline period (1951–1980). The anomaly method removes systematic biases such as station location or instrument changes. The global average is computed by weighting temperature anomalies by the area of grid cells, with adjustments for spatial coverage gaps using spatial interpolation. The resulting time series is a widely accepted measure of global climate change.

Why annual values fluctuate

Year-to-year variations in global temperature are primarily driven by internal climate variability, particularly the El Niño–Southern Oscillation (ENSO). El Niño events (warm phase) tend to increase global temperatures, while La Niña events (cool phase) suppress them. Other factors include volcanic eruptions, which can cause short-term cooling, and solar variability. The long-term trend, however, is dominated by increasing greenhouse gas concentrations. Truncating the y-axis can exaggerate these natural fluctuations, making them appear as dramatic spikes or dips rather than noise around a rising trend.

Regional variation

Global temperature anomalies mask significant regional differences. The Arctic, for example, has warmed at roughly twice the global average rate. The table below shows 2023 temperature anomalies by latitude band, relative to the 1951–1980 baseline, based on NASA GISS data:

When visualizing regional trends, truncated axes can similarly distort the perceived magnitude of warming, especially in high-latitude regions where anomalies are larger.

Meaning and limitations

The global temperature anomaly is a key indicator of climate change, but its visual representation can be manipulated. Truncating the y-axis—starting it at a value higher than zero or the baseline—can make a gradual trend appear as a sudden spike, exaggerating the rate of change. Conversely, truncating the top of the axis can downplay extreme values. In environmental communication, such practices can mislead the public and policymakers. Best practices for accurate visualization include: using a baseline that represents a meaningful reference period (e.g., pre-industrial), showing the full range of data, and clearly labeling axes. The temperature record itself has limitations: spatial coverage is uneven, particularly in polar regions and earlier periods, and the baseline choice affects the anomaly values. Users should always check the axis scales and baseline when interpreting charts.

Next expected update

NASA GISS typically releases the annual global temperature anomaly for the previous year in mid-January. The 2024 full-year figure is expected around 10–15 January 2025. Monthly updates are also available with a lag of approximately two weeks.

Downloadable chart or table

The table below provides annual global temperature anomalies from 2010 to 2023 (NASA GISS). The full dataset can be downloaded from NASA GISTEMP.