Global warming is the long-term increase in Earth’s surface temperature. Human greenhouse-gas emissions are the principal cause of the warming observed in the modern era. Understanding the evidence requires reading the quantity, geographic coverage and time period together.
This guide helps a reader make sense of a temperature chart or climate projection. Keep the source page open alongside the graphic: its notes explain the baseline, data processing and uncertainty that a cropped image can omit.
Look at connected lines of evidence
NASA’s evidence overview brings together surface temperature, ocean warming, ice changes and other observations. These measure different parts of the climate system. Their combined interpretation is stronger than selecting a single unusually warm or cold day.
The IPCC’s 2023 synthesis assessment attributes global warming to human activities, principally greenhouse-gas emissions. Its approximately 1.1°C figure refers to the 2011–2020 average relative to 1850–1900. Retain both periods when using that assessment number; it is not a reading for an individual day or the latest calendar year.
Greenhouse gases affect outgoing infrared radiation and Earth’s energy balance. The response includes warming oceans and changes in ice as well as surface air temperatures. Local weather still varies within the changing climate.
Read a chart before interpreting its slope
NASA GISS provides temperature graphs and underlying data. Open a chosen graph and read its caption before comparing it with another service.
On a narrow screen, scroll the table sideways. Keyboard users can focus it and use the arrow keys.
| Element | What to identify | Why it matters |
|---|---|---|
| Quantity and unit | Temperature anomaly in °C, or another stated measure. | An anomaly is a difference from a reference average. |
| Baseline | The years used for the reference. | Different baselines shift the chart vertically. |
| Coverage | Global, land-only, ocean or local. | Different areas can warm at different rates. |
| Time step | Month, year or multi-year average. | A short interval can emphasize variability. |
| Processing | Smoothing, missing data and uncertainty notes. | The plotted line is a defined data product. |
Trace a point to its axes, then write one sentence naming all five elements. Check whether the newest point represents a complete period before comparing it with annual values.
Work through an anomaly example
The GISTEMP FAQ explains anomalies and baseline choice. Changing a fixed reference shifts the values; it preserves the difference between two temperatures in the same series.
Read projections as conditional results
The IPCC assessment’s scenario explanation describes pathways with assumptions about future emissions and development. A projection asks how the climate could respond under those conditions. Read the pathway name, period, region and uncertainty range together.
Compare like quantities: a global multi-decade average and a local annual extreme are different outcomes. A central estimate is only part of the result; ranges reflect specified uncertainties and model or scenario choices. Check what the source says the band represents.
For a local decision, seek an appropriate regional assessment and the variable needed for that decision, such as heavy rainfall or heat exposure. A global temperature curve supplies context but cannot design a drainage system or establish a site-specific risk.
Keep the evidence attached when sharing
Save the source URL, dataset version, retrieval date and full caption. Retain axes and units when presenting a chart. Distinguish an observed value, a smoothed trend and a projection in your wording.
When two graphics appear to disagree, compare baselines, geographic coverage, averaging periods and updates before attributing the difference to an error. If those match, consult their methods and uncertainty notes.
Understanding a climate indicator is separate from calculating an organization’s footprint. For the latter, use the emissions inventory guide; for choosing a bounded action, use the sustainability planning guide.
Climate chart reading worksheet
Climate chart reading worksheet — plain-text download. Save a copy and fill it in with your own information. It includes instructions, example assumptions and source references.