ICESat-2¶

ICESat-2 L4 Monthly Gridded Sea Ice Thickness (IS2SITMOGR4, Version 4) · ATLAS/ICESat-2 L3B Daily and Monthly Gridded Sea Ice Freeboard (ATL20, Version 5), with the along-track products ATLAS/ICESat-2 L3A Sea Ice Height (ATL07) and L3A Sea Ice Freeboard (ATL10), Version 7

Available

At a glance¶

Note on period: ICESat-2 science data begin on 14 October 2018. As of 2026-10-12 (NASA CMR), the latest granules are ATL07/ATL10 v7 for 2026-07-20, ATL20 v5 for February 2026 (monthly), IS2SITMOGR4 v4 for April 2026 (monthly) and Quick Look ATL10QL for 2026-10-08. IS2SITMOGR4 covers Arctic winters only: November 2018 – April 2019 for the first winter, then September through April each year. There are no science data during the spacecraft safe-mode periods of 4–16 April 2022 and 10 May – 21 June 2024 (with additional partial gaps in June–July 2019).

Overview¶

The Advanced Topographic Laser Altimeter System (ATLAS), a photon-counting lidar on NASA's ICESat-2, measures the heights of sea ice and of leads (open water between floes). From these, the products derive total freeboard (the height of the snow surface above local sea level) and, with assumptions about snow depth and density, sea ice thickness. Most research use relies on the monthly 25 km gridded thickness IS2SITMOGR4 (Arctic winter, with NESOSIM snow loading) and the daily and monthly 25 km gridded freeboard ATL20 (both hemispheres, all seasons), both built from the along-track sea ice height (ATL07) and freeboard (ATL10). Because the laser ranges to the snow surface rather than the ice surface sensed by radar altimeters such as CryoSat-2, the assumed snow loading largely determines the retrieved thickness.

What changes with the successor¶

ICESat-2 Release 006 → ICESat-2 Release 007¶

Transition: 2025-10

The along-track products moved to v7 with a new freeboard reference surface calculation, the gridded freeboard and thickness products followed with new versions, and all previous versions were retired during 2026.

Sources: NSIDC announcement — ATL07, ATL10 and ATL12 v7 release (2025-10-07) · ATL10 v7 user guide (version history) · ATL07 v7 user guide (version history) · ATL20 v5 user guide (version history) · IS2SITMOGR4 v4 user guide (version history)

Available variables¶

Names as given in the NSIDC user guides and data dictionaries (IS2SITMOGR4 v4, ATL20 v5, ATL07/ATL10 v7). IS2SITMOGR4 uses netCDF-4 variables; the ATL products use HDF5 paths, where gtx stands for one of the six beam groups gt1l–gt3r.

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

Monthly gridded sea ice thickness (IS2SITMOGR4, Arctic, September–April)¶

Monthly gridded freeboard, snow and ancillary fields (IS2SITMOGR4)¶

Daily and monthly gridded freeboard (ATL20, both hemispheres)¶

Along-track data (ATL07 sea ice height · ATL10 freeboard)¶

The full variable list is in the official documentation.

How to get the data¶

• NSIDC — IS2SITMOGR4 v4 (monthly gridded sea ice thickness) — Earthdata Search / earthaccess (Python) / HTTPS / NASA Earthdata Cloud (AWS us-west-2)
  • Requires a free NASA Earthdata Login. One netCDF-4 file per month (IS2SITMOGR4_01_YYYYMM_007_004.nc, 01 = Northern Hemisphere) with a browse PNG. The CMR version is "4". No Southern Hemisphere (02) files exist yet.
• NSIDC — ATL20 v5 (daily and monthly gridded freeboard) — Earthdata Search / earthaccess / HTTPS / NASA Earthdata Cloud
  • HDF5; one file per hemisphere (01 north, 02 south) and month, holding the monthly composite and the daily composites. If a file is reprocessed under the same name, use the one with the highest trailing revision number. A rolling 30-day gridded Quick Look (ATL20QL v5, built from ATL10QL) was released on 2026-06-16.
• NSIDC — ATL10 v7 (along-track freeboard) · ATL07 v7 (along-track sea ice height) — Earthdata Search / earthaccess / NSIDC Data Access Tool (subsetting) / Harmony
  • Up to 32 HDF5 granules per day (per-hemisphere orbit segments). ATL07 is at https://nsidc.org/data/atl07/versions/7. The 3–5 day Quick Look products (ATL07QL, ATL10QL v7) are deleted once the standard files are out, so use the standard products for the research record.
• NASA Earthdata Search — Web search and download
  • The link opens the IS2SITMOGR4 v4 collection (C3878683537-NSIDC_CPRD). ATL20 v5 is C3825009052-NSIDC_CPRD.

Example code¶

Install with pip install earthaccess xarray h5py netCDF4. On the first login() you can enter your Earthdata credentials and save them to ~/.netrc. Monthly thickness (IS2SITMOGR4) opens with xarray; ATL20 uses HDF5 groups, so read it with h5py.

import earthaccess
import h5py
import numpy as np
import xarray as xr

earthaccess.login()  # NASA Earthdata Login

# 1) Monthly gridded sea ice thickness (IS2SITMOGR4 v4) — March 2026
res = earthaccess.search_data(short_name="IS2SITMOGR4", version="4",
                              temporal=("2026-03-01", "2026-03-31"))
files = earthaccess.download(res, "./is2sitmogr4")
nc = [f for f in files if str(f).endswith(".nc")][0]
ds = xr.open_dataset(nc)
sit = ds["ice_thickness"].squeeze()            # m, NESOSIM snow loading
print(float(sit.mean()), float(ds["mean_day_of_month"].mean()))

# 2) Monthly gridded freeboard (ATL20 v5) — February 2026, Northern Hemisphere (01)
res = earthaccess.search_data(short_name="ATL20", version="005",
                              temporal=("2026-02-01", "2026-02-28"))
files = earthaccess.download(res, "./atl20")
h5 = [f for f in files if "ATL20-01_" in str(f) and str(f).endswith(".h5")][0]
with h5py.File(h5, "r") as h:
    v = h["monthly/mean_fb"]
    fb = v[:].astype(float)
    fill = v.attrs.get("_FillValue")
    if fill is not None:
        fb[fb == fill] = np.nan
    nseg = h["monthly/n_segs"][:]
    print(list(h["daily"].keys())[:3])        # daily composite groups
print(np.nanmean(fb))

Practical tips¶

• ICESat-2 measures total freeboard, i.e. to the snow surface. Snow depth and density are the largest error sources in the thickness conversion, so before comparing with CryoSat-2 thickness (radar, ice-surface freeboard) make sure both use the same snow product. The IS2SITMOGR4 developers recommend the NESOSIM-based ice_thickness and advise caution with the alternative thickness variables.
• The SnowModel-LG variables in IS2SITMOGR4 (*_sm_e5, *_sm_m2) are only available through July 2021 (user guide footnote). They are empty for recent winters, so do not mix them with the primary variable.
• OSI SAF does not provide ice type in September, so all ice is treated as multiyear ice that month. In April 2022 the safe mode removed the first part of the month (4–11 April per Known Issues), biasing that monthly mean toward the second half. Sampling dates differ between grid cells, so always check mean_day_of_month in regional comparisons.
• Near the ice edge, some 10 km segments still use a reference surface lowered by tens of centimetres by wave troughs, which inflates freeboard (ATL07/ATL10 and ATL20 Known Issues). The multi-beam (swath) freeboard in ATL10 is disabled because of beam alignment issues; only the per-beam values (beam_fb_height) are valid.
• Do not mix versions. Version 7 changed how ATL10 builds the reference surface (from a static 10 km value to interpolation between neighbouring reference surfaces), so freeboards differ from v6, and v6, ATL20 v4 and IS2SITMOGR4 v3 files can no longer be downloaded. Since 2026-01-15 the ATL07/ATL10 sea ice concentration input has been the AMSR2-based NRT CDR (G10016 v4), which introduces a very small (≤ 0.4%) discontinuity at the ice edge.
• ATL20 is produced for summer months too, but clouds sharply reduce coverage and surface melt makes ATL10 retrievals difficult (IS2SITMOGR4 Known Issues). IS2SITMOGR4 covers September–April only; summer thickness is still at the "initial assessment" research stage (Petty, Cabaj & Landy, 2025). North of 88°N lies the pole hole not covered by the orbit.

Notices and change history¶

References for your paper¶

Dataset citation¶

Monthly gridded sea ice thickness (IS2SITMOGR4, Version 4) — DOI 10.5067/TXDHDJ1JT0CG

@misc{petty2025is2sitmogr4v4,
  author    = {Petty, A. A. and Kurtz, N. and Kwok, R. and Markus, T. and Neumann, T. A. and Keeney, N. and Cabaj, A.},
  title     = {{ICESat-2} {L4} Monthly Gridded Sea Ice Thickness ({IS2SITMOGR4}, Version 4)},
  publisher = {NASA National Snow and Ice Data Center Distributed Active Archive Center},
  address   = {Boulder, Colorado USA},
  year      = {2025},
  doi       = {10.5067/TXDHDJ1JT0CG}
}

Daily and monthly gridded freeboard (ATL20, Version 5) — DOI 10.5067/ATLAS/ATL20.005

@misc{kwok2025atl20v5,
  author    = {Kwok, R. and Kurtz, N. and Wimert, J. and Petty, A. A. and Cunningham, G. and Markus, T. and Hancock, D. and Ivanoff, A. and Bagnardi, M. and Herzfeld, U. C. and Trantow, T. and {the ICESat-2 Science Team}},
  title     = {{ATLAS/ICESat-2} {L3B} Daily and Monthly Gridded Sea Ice Freeboard ({ATL20}, Version 5)},
  publisher = {NASA National Snow and Ice Data Center Distributed Active Archive Center},
  address   = {Boulder, Colorado USA},
  year      = {2025},
  doi       = {10.5067/ATLAS/ATL20.005}
}

Along-track freeboard (ATL10, Version 7) — DOI 10.5067/ATLAS/ATL10.007

@misc{kwok2025atl10v7,
  author    = {Kwok, R. and Petty, A. A. and Cunningham, G. and Markus, T. and Hancock, D. and Ivanoff, A. and Wimert, J. and Bagnardi, M. and Kurtz, N. and {the ICESat-2 Science Team}},
  title     = {{ATLAS/ICESat-2} {L3A} Sea Ice Freeboard ({ATL10}, Version 7)},
  publisher = {NASA National Snow and Ice Data Center Distributed Active Archive Center},
  address   = {Boulder, Colorado USA},
  year      = {2025},
  doi       = {10.5067/ATLAS/ATL10.007}
}

Along-track sea ice height (ATL07, Version 7) — DOI 10.5067/ATLAS/ATL07.007

@misc{kwok2025atl07v7,
  author    = {Kwok, R. and Petty, A. A. and Cunningham, G. and Markus, T. and Hancock, D. and Ivanoff, A. and Wimert, J. and Bagnardi, M. and Kurtz, N. and {the ICESat-2 Science Team}},
  title     = {{ATLAS/ICESat-2} {L3A} Sea Ice Height ({ATL07}, Version 7)},
  publisher = {NASA National Snow and Ice Data Center Distributed Active Archive Center},
  address   = {Boulder, Colorado USA},
  year      = {2025},
  doi       = {10.5067/ATLAS/ATL07.007}
}

Reference papers¶

ICESat-2 freeboard-to-thickness method (recommended in the IS2SITMOGR4 user guide) — DOI 10.1029/2019JC015764

@article{petty2020is2sit,
  author  = {Petty, Alek A. and Kurtz, Nathan T. and Kwok, Ron and Markus, Thorsten and Neumann, Thomas A.},
  title   = {Winter {Arctic} Sea Ice Thickness From {ICESat-2} Freeboards},
  journal = {Journal of Geophysical Research: Oceans},
  volume  = {125},
  number  = {5},
  pages   = {e2019JC015764},
  year    = {2020},
  doi     = {10.1029/2019JC015764}
}

Freeboard and snow loading upgrades and first three winters (recommended in the IS2SITMOGR4 user guide) — DOI 10.5194/tc-17-127-2023

@article{petty2023is2sit,
  author  = {Petty, Alek A. and Keeney, Nicole and Cabaj, Alex and Kushner, Paul and Bagnardi, Marco},
  title   = {Winter {Arctic} sea ice thickness from {ICESat-2}: upgrades to freeboard and snow loading estimates and an assessment of the first three winters of data collection},
  journal = {The Cryosphere},
  volume  = {17},
  number  = {1},
  pages   = {127--156},
  year    = {2023},
  doi     = {10.5194/tc-17-127-2023}
}

ATL07/ATL10 surface height and freeboard — early results — DOI 10.1029/2019JC015486

@article{kwok2019icesat2fb,
  author  = {Kwok, R. and Markus, T. and Kurtz, N. T. and Petty, A. A. and Neumann, T. A. and Farrell, S. L. and Cunningham, G. F. and Hancock, D. W. and Ivanoff, A. and Wimert, J. T.},
  title   = {Surface Height and Sea Ice Freeboard of the {Arctic} {Ocean} From {ICESat-2}: Characteristics and Early Results},
  journal = {Journal of Geophysical Research: Oceans},
  volume  = {124},
  number  = {10},
  pages   = {6942--6959},
  year    = {2019},
  doi     = {10.1029/2019JC015486}
}

All-season (including summer) ICESat-2 thickness — initial assessment (cited in the IS2SITMOGR4 user guide) — DOI 10.1017/jog.2025.10119

@article{petty2026allseason,
  author  = {Petty, Alek and Cabaj, Alex and Landy, Jack C.},
  title   = {Initial assessment of all-season {Arctic} sea ice thickness from {ICESat-2}},
  journal = {Journal of Glaciology},
  volume  = {72},
  pages   = {e4},
  year    = {2026},
  doi     = {10.1017/jog.2025.10119}
}

Acknowledgements¶

NASA asks that the data sets used be cited in the text and listed in the references, and NSIDC asks for a data citation that includes the DOI, version, date accessed and any subset used. If you use IS2SITMOGR4, also cite Petty et al. (2020, 2023), and, when using the alternative snow (NESOSIM, SnowModel-LG) or ice density variables, the input data references given in the user guide.

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¶

• IS2SITMOGR4 v4 user guide — variables, snow loading, interpolation, file naming
• IS2SITMOGR4 and IS2SITDAT4 Known Issues (updated 2025-11-17)
• ATL20 v5 user guide — grids and HDF5 structure
• ATL07/ATL10/ATL20/ATL21 Algorithm Theoretical Basis Document (ATBD), Release 007
• ATL07/ATL10 v7 Known Issues
• ATL20/ATL21 Known Issues
• ICESat-2 data gaps list (xlsx)
• NSIDC ICESat-2 news and announcements