Why Truncated Axes Can Distort Environmental Trends

Current figure

The global average surface temperature anomaly for the calendar year 2024 was 1.28°C above the 1951–1980 baseline, as reported by NASA’s Goddard Institute for Space Studies (GISS) in its GISTEMP v4 dataset. This figure represents the departure of the Earth’s combined land and ocean surface temperature from the long-term average for the baseline period.

Measurement date

The current figure refers to the full calendar year 2024 (January through December). NASA GISS updates its global temperature record monthly, with the annual summary typically released in mid-January of the following year. The 2024 annual value was published on 10 January 2025.

Previous figure

The global temperature anomaly for 2023 was 1.17°C above the 1951–1980 baseline. The year-on-year change from 2023 to 2024 is therefore an increase of +0.11°C, equivalent to a rise of approximately 9.4% relative to the 2023 anomaly. Both 2023 and 2024 are the two warmest years in the instrumental record, which extends back to 1880.

Long-term trend

The long-term global temperature trend shows a clear and sustained warming since the late 19th century. According to NASA GISS, the average rate of warming since 1880 is about 0.08°C per decade, but this rate has accelerated to more than 0.18°C per decade since 1981. The ten warmest years on record have all occurred since 2014, with 2024 being the warmest. The table below presents annual global temperature anomalies for the most recent decade.

Source: NASA GISS GISTEMP v4.

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 meteorological stations with sea surface temperatures measured by ships and buoys, adjusted for historical changes in measurement methods. The data are publicly available at https://data.giss.nasa.gov/gistemp/. Other major global temperature datasets, such as those from NOAA and the UK Met Office Hadley Centre (HadCRUT5), show very similar long-term trends.

Methodology

NASA GISS calculates temperature anomalies by comparing current temperatures to a baseline average for the same location and time of year. The baseline period used is 1951–1980, a relatively stable climatic interval before the most pronounced recent warming. Anomalies are computed for each station, then gridded and averaged globally, with adjustments for urban heat island effects and station inhomogeneities. The resulting global mean anomaly is an area-weighted average of land and ocean surface temperature departures. The uncertainty in the annual global mean is approximately ±0.05°C (95% confidence interval), primarily due to incomplete spatial coverage and measurement errors.

Why annual values fluctuate

Year-to-year variations in global temperature are driven by natural climate phenomena superimposed on the long-term warming trend. The most significant short-term driver is the El Niño–Southern Oscillation (ENSO). El Niño events, such as the strong one that peaked in late 2023 and persisted into early 2024, temporarily boost global temperatures by releasing heat from the tropical Pacific Ocean. La Niña events have a cooling effect. Other factors include volcanic eruptions, which can inject sunlight-reflecting aerosols into the stratosphere (e.g., the 2022 Hunga Tonga eruption had a small warming effect due to water vapour), and variations in solar activity. These natural fluctuations do not alter the underlying long-term trend driven by increasing greenhouse gas concentrations.

Regional variation

Warming is not uniform across the globe. The Arctic has warmed at roughly twice the global average rate, a phenomenon known as Arctic amplification. Land areas generally warm faster than oceans. The table below shows 2024 temperature anomalies for selected regions relative to the 1951–1980 baseline, based on NASA GISS data.

Source: NASA GISS GISTEMP v4, 2024 annual means.

Meaning and limitations

The global temperature anomaly is a key indicator of climate change, but its visual representation can be easily distorted. A common misleading practice is truncating the y-axis of a time-series chart. When the vertical axis does not start at zero, the slope of the line appears steeper, exaggerating the perceived rate of change. For example, a chart showing the global temperature anomaly from 1880 to 2024 with a y-axis ranging from 0.5°C to 1.5°C will make the recent warming look dramatically sharper than a chart with an axis starting at 0°C. While truncation can be useful to highlight small variations in a stable series, for environmental trends where the absolute magnitude matters, a zero baseline provides the most honest visual context. Viewers should always check axis labels and consider the full range of the data. The temperature anomaly itself also has limitations: it does not capture regional extremes, precipitation changes, or impacts on ecosystems. It is a global average that smooths out local variability.

Next expected update

NASA GISS releases monthly global temperature updates around the middle of each month. The next annual summary for the full calendar year 2025 is expected in January 2026. Preliminary monthly data for 2025 will be available on a rolling basis at the NASA GISS website.

Downloadable chart or table

The table below provides annual global temperature anomalies for the entire instrumental record in a compact format. The full dataset, including monthly and seasonal values, can be downloaded from the NASA GISS GISTEMP v4 data portal at https://data.giss.nasa.gov/gistemp/.

Note: Values are decadal averages for the year shown (e.g., 1880 represents 1880–1889). Source: NASA GISS GISTEMP v4.