NCDC Frequently Asked Questions

The NOAA National Climatic Data Center (NCDC) is one of the world's premier centers for archiving, processing, and researching climate data. As such, NCDC fields hundreds of questions regarding the data: how it is processed, how it can be accessed, and what it means. If you have a question about NCDC, climate data, or climatology, an answer may reside in our "Frequently Asked Questions" (FAQs) below. If not, please feel free to contact us. We're adding questions to this site all the time.

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Global Climate Change Questions
- Is the climate warming?
  - Globally, annually–averaged surface temperature has increased by just under 1 °C since the late 1800s. The rate of increase over the past 50 years is nearly twice that of the past century. However, this warming trend is not uniform across the planet. [ Read More ]
- How do we know the Earth's climate is warming?
  - Millions of temperature observations have been made at thousands of locations around Earth over more than two centuries. These observations are processed, quality controlled, and then used to examine their behavior over time. [ Read More ]
- How do we know humans are the primary cause of the warming?
  - While natural variability does play an important role in climate change, a large body of evidence supports the conclusion that human activity is the primary driver of the current warming. [ Read More ]
- Are El Niños related to global warming?
  - El Niños are not caused by global warming. They have occurred for thousands of years. Whether global climate warming will affect future El Niños remains under investigation. [ Read More ]
- Is the hydrological cycle (evaporation and precipitation) changing?
  - Averaged globally, precipitation shows a slight, but not significant upward trend. Regionally, higher latitudes of the Northern Hemisphere, southern South America, and northern Australia have seen greater increases, while decreases have occured in the African and Asian tropics. [ Read More ]
- Is the atmosphere/oceanic circulation changing?
  - Atmospheric and oceanic circulations change on many scales in time and space. Scientists noted an abrupt change in El Niño behavior in 1976/77 that has persisted to this day. However, it is unclear whether any of these shifts are related to global warming. [ Read More ]
- Is the climate becoming more variable or extreme?
  - While there does not appear to be any significant change in interannual temperature extremes, there is substantial decrease in intra–annual variability. This includes a trend toward fewer extremely low minimum temperatures. Precipitation has shown a tendancy toward increasing dry and wet spells and increasing drought worldwide. [ Read More ]
- How important are these changes in a longer–term context?
  - Many of the climate changes we are currently seeing are the greatest in the past 1000 years or more. Model projections of climate change over the next 100 years estimate warming that is unprecedented in the climate record we have collected to date. [ Read More ]
- Is sea level rising?
  - Globally–averaged sea level has been rising at a rate of about 1.7 mm per year. [ Read More ]
- Can the observed changes be explained by natural variability, including changes in solar output?
  - Natural variability plays a substantial role in climate change, but not all variability in climate has been accounted for by known natural variations. [ Read More ]
- What about the future?
  - Most climate models continue to forecast that the climate will continue to warm over the course of the 21st Century. However, such change depends upon many factors influencing a very complex environmental system. [ Read More ]
The Greenhouse Effect
- What is the greenhouse effect, and is it affecting our climate?
  - The greenhouse effect describes the ability of the atmosphere to keep heat energy released by the earth from being immediately lost to space. It keeps our lower atmosphere about 60 °F warmer than it otherwise would be. [ Read More ]
- What are greenhouse gases?
  - Greenhouse gases are gases that, in the atmosphere, allow shortwave solar radiation to pass through to reach Earth's surface, but which intercept the longwave heat energy radiated by the warm Earth's surface. [ Read More ]
- Are greenhouse gases increasing?
  - Human activity has been increasing the concentration of greenhouse gases in the atmosphere. Current amounts of many greenhouse gases are far greater than they have been in the past. [ Read More ]
Climate Change Data
- How reliable is the U.S. surface temperature record?
  - The U.S. surface temperature record is very reliable. All data undergoes strict quality assurance, and a recent study shows no evidence that variable station exposure artificially inflates the trend in U.S. temperatures. [ Read More ]
- What datasets are used in calculating the average global temperature anomaly?
  - Land surface temperatures come from the Global Historical Climate Network (GHCN) and sea surface temperatures are from the extended reconstructed sea surface temperature (ERSST) analysis. [ Read More ]
- What version of the ERSST analysis is currently being used?
  - ERSST version 3b is currently in use. [ Read More ]
- When was ERSST version 3b first implemented?
  - Transition to version 3b of ERSST occurred in November 2008, and the NCDC Climate Monitoring Branch first used the merged land and ocean dataset operationally with the July 2009 State of the Climate Report.
- How is the average global temperature anomaly time series calculated?
  - The global temperature anomaly time series is calculated from a blended land and ocean dataset that consists of gridded monthly average temperature anomalies. [ Read More ]
- Why do some of the products use different reference periods?
  - Most anomalies products reference a 1971–2000 base period because this is the most recent "normals" period and is a period with a great deal of historical data coverage. Some products reference the 20th Century average for conceptual simplicity. The choice of base reference periods has no bearing on the between year changes nor on the trend. [ Read More ]
- How often and when is the global average temperature dataset updated?
  - The dataset is updated every month. Data for a month are typically made available by the 15th of the following month.
- What is the difference between the gridded dataset and the index values?
  - Index values are averages of the gridded dataset values. [ Read More ]
- Is it appropriate to use normals for predictive purposes?
  - Normals are often used as a base value against which temperatures in subsequent years may be compared. [ Read More ]
- What is the difference between Cooperative and First–Order stations?
  - Each are weather observation networks maintained by the Federal Government. First–Order stations are generally found at larger airports, while cooperative stations are operated largely by volunteer citizen observers. [ Read More ]
- If measuring equipment changes at a site, does that change the records? Does it change the normals?
  - Unless identified and accounted for, changes in instrumentation can affect the observational record and any values, such as normals, that are calculated from them. [ Read More ]
- How are degree day normals computed?
  - Beginning with the 1971–2000 normals, degree day normals were calculated using a spline fit approach. [ Read More ]
Climate Definitions
- What is a temperature anomaly?
  - A temperature anomaly is the difference between a temperature value and a reference baseline. The baseline value is often a 30–year, centennial, or period–of–record average. [ Read More ]
- Why use temperature anomalies and not absolute temperature measurements?
  - Anomalies allow for the comparison and combination of data from different datasets by standardizing values against a common baseline. Anomalies also reveal important behavioral traits within the data. [ Read More ]
- What is a climate normal?
  - Normals are generally averages of climate elements such as temperature or precipitation over a 30–year period. [ Read More ]
Briefings on Climate Change
- NOAA's Temperature Records
  - A Foundation for Understanding Global Warming - Dr. Thomas R. Karl [ Read More ]

Answers to questions related to climate data access at NCDC may be found on NCDC's data access and ordering FAQ page.

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