Sea Ice Index¶

NOAA/NSIDC Sea Ice Index (G02135, Version 4)

Available

At a glance¶

Note on period: Daily data from 1978-10-26 and monthly data from 1978-11; as of 2026-10-12 updated daily, one day behind (per the data set page and Arctic Sea Ice News; the archive listing itself was not opened). Input is NSIDC-0051 (SMMR, SSM/I, SSMIS) for 1978-10 to 2024-12 and NSIDC-0803 (AMSR2) from 2025-01.

Overview¶

The Sea Ice Index provides a quick look at Arctic- and Antarctic-wide changes in sea ice: daily and monthly sea ice extent and area indices computed by NSIDC from passive microwave sea ice concentration, together with concentration and extent images. It uses NASA Team concentrations (NSIDC-0051) from 1978 and continues with AMSR2-based NSIDC-0803 concentrations from 2025, and provides anomaly and trend images relative to the 1981–2010 reference period along with CSV, GeoTIFF and shapefile products. Most figures such as "record-low Arctic sea ice" in the media and reports come from this data set.

What changes with the successor¶

Sea Ice Index V3 → Sea Ice Index V4¶

Transition: 2025-07

In response to the loss of access to DMSP SSMIS data, the input sea ice concentration from 2025-01-01 onwards switched from SSMIS-based products (NSIDC-0081, NSIDC-0051) to AMSR2-based NSIDC-0803. Data before 2025 were not reprocessed but copied from V3 (only the file names changed to v4.0), and the distinction between near-real-time and final data was removed. V3 was retired when V4 was released.

Sources: NSIDC — User Notice: Sea Ice Index Transitioning to Version 4 with AMSR2 Data (2025-07-23) · NSIDC — New version release: NOAA/NSIDC Sea Ice Index, Version 4 (2025-07-31) · NSIDC Special Report 28 — Sea Ice Index Version 4 Analysis (July 2025) · G02135 V4 User Guide

Available variables¶

CSV column names and image/GeoTIFF types as given in the User Guide. Extent and area are in million km², and missing values are -9999. Images (PNG) and GeoTIFFs are named h_YYYYMMDD_concentration_vX.x.tif (daily) and h_YYYYMM_concentration_vX.x.tif (monthly), with the same pattern for extent (h = N or S).

instantaneous mean accumulated maximum minimum static — instantaneous: value at the time stamp · mean/accumulated/maximum/minimum: over the indicated interval · static: does not change with time

Daily index (h_seaice_extent_daily_v4.0.csv)¶

Monthly index (h_mm_extent_v4.0.csv, one file per month)¶

Gridded images and GeoTIFFs¶

Climatology and median ice edge (1981–2010)¶

The full variable list is in the official documentation.

How to get the data¶

• NSIDC data set page — G02135 Version 4 — Web documentation / User Guide / citation
  • Links to the User Guide, Special Report 28 (V4 analysis) and the overview of the sea ice analysis spreadsheets.
• NOAA@NSIDC HTTPS archive — G02135 — HTTPS (browser, wget, Python/R) No login needed
  • No login needed. Under north/ and south/ there are daily/ (data, geotiff, images) and monthly/ (data, geotiff, etc.) folders and monthly shapefile folders (shp_extent, shp_median); seaice_analysis/ holds the analysis spreadsheets. Check the directory listing for the exact layout.
• Sea Ice Index Daily and Monthly Image Viewer — Web image viewer / compare and animation tools
  • Shows daily and monthly extent, concentration, anomaly and trend images. The data and image archive is at https://nsidc.org/data/seaice_index/data-and-image-archive.

Example code¶

Reads the Northern Hemisphere daily extent CSV straight from HTTPS without logging in. Column names may have surrounding spaces and there may be a units row, so the code cleans them up.

import pandas as pd

url = "https://noaadata.apps.nsidc.org/NOAA/G02135/north/daily/data/N_seaice_extent_daily_v4.0.csv"
df = pd.read_csv(url, skipinitialspace=True)
df.columns = df.columns.str.strip()
df = df[pd.to_numeric(df["Year"], errors="coerce").notna()]      # drop the units row
df["date"] = pd.to_datetime(df[["Year", "Month", "Day"]].astype(int))
df["Extent"] = df["Extent"].astype(float)                         # million km²

# From 2025 the input is AMSR2 (NSIDC-0803 in the Source Data column)
print(df.groupby(df["date"].dt.year)["Extent"].min().tail())

Practical tips¶

• NSIDC recommends the monthly products, not daily values, for long-term trend analysis: daily concentrations are more affected by weather and uncertainty, and the daily ice edge is not accurate enough for operational purposes such as navigation.
• From 2025-01-01 the input changed from SSMIS to AMSR2, which generally gives smaller extents (in the first half of 2025, mean differences of 0.04–0.05 and maxima below 0.15–0.2 million km²). State the sensor change when discussing "record low" rankings from 2025 or trends since 1979. For the same reason, values from 2025 onwards do not exactly match other AMSR2-based records such as the NOAA/NSIDC CDR (G02202 V6).
• Extent and area are different. Extent sums the full area of cells at or above 15 percent; area sums the concentration-weighted ice area. The Arctic pole hole is included in extent and excluded from area. Because the pole hole differs by sensor (SMMR 1.19, SSM/I 0.31, SSMIS 0.029, AMSR2 0.064 million km²), Arctic area time series can show steps at sensor changes.
• Values depend on the 15 percent threshold and the NASA Team algorithm, so do not compare absolute extents directly with those from other agencies (OSI SAF, JAXA, etc.). The anomaly and trend reference period is 1981–2010.
• NSIDC-0051 (SSMIS-based final concentration) and NSIDC-0081 (near-real-time) stopped updating or were retired in 2026, but Sea Ice Index V4 has used NSIDC-0803 (AMSR2) since 2025 and is unaffected. The data set has, however, been at the Basic Level of Service since May 2025, so error fixes and replies to questions may be slow.

Notices and change history¶

References for your paper¶

Dataset citation¶

Sea Ice Index Version 4 (G02135) — DOI 10.7265/a98x-0f50

@misc{nsidc_g02135_v4,
  author    = {Fetterer, F. and Knowles, K. and Meier, W. N. and Savoie, M. and Windnagel, A. K. and Stafford, T.},
  title     = {Sea Ice Index ({G02135}, Version 4)},
  year      = {2025},
  publisher = {National Snow and Ice Data Center},
  address   = {Boulder, Colorado USA},
  doi       = {10.7265/a98x-0f50}
}

Reference papers¶

Version 4 analysis report (impact of the AMSR2 transition)

Acknowledgements¶

NSIDC requires citation of the data set as a condition of use. Give the version (V4) and DOI, the hemisphere and period used and the access date, and if you use values from 2025 onwards, state that they are based on AMSR2 input (NSIDC-0803).

Information on the datasets used in this study was compiled with the help of the Korean Climate Data Navigator (KCDN) (Sim, 2026; https://doi.org/10.5281/zenodo.23300923).

Links¶

• G02135 V4 User Guide
• NSIDC Special Report 28 — Sea Ice Index Version 4 Analysis
• Sea Ice Analysis Spreadsheets overview
• Interpretation resources: sea ice trends and anomalies
• Arctic Sea Ice News & Analysis (NSIDC)