What happened?

Adding new variables to a zarr dataset with to_zarr() always removes the existing global attributes. New global attributes in the source dataset are not always added to the destination dataset depending on how to_zarr() is called.

What did you expect to happen?

I would expect that existing global attributes would always be preserved. If there are new global attributes I would expect them to be added to the existing global attributes instead of replacing all existing global attributes.

Minimal Complete Verifiable Example

```Python import xarray as xr from pyproj import CRS

local_zarr = 'sample.zarr'

ds_sample = xr.tutorial.load_dataset("air_temperature")

Make a local copy

ds_sample.to_zarr(local_zarr, mode='w') ds_sample = xr.open_dataset(local_zarr, engine='zarr', backend_kwargs={'consolidated':True}, chunks={}, decode_coords=True)

Create CRS metadata

crs_meta = CRS.from_epsg(4326).to_cf()

ds_new = xr.Dataset(data_vars={"crs": ([], 1, crs_meta)}) ds_new.attrs['note'] = 'please add this'

Add all variables from ds_new to the zarr

NOTE: This adds the new global attribute but also removes

all existing global attributes

ds_new.to_zarr(local_zarr, mode='a')

Add selected variable(s) to zarr dataset

NOTE: This does not copy new global attributes

and removes all existing global attributes

ds_new['crs'].to_zarr(local_zarr, mode='a')

Re-open local zarr store

ds_sample = xr.open_dataset(local_zarr, engine='zarr', backend_kwargs={'consolidated':True}, chunks={}, decode_coords=True)

ds_sample ```

MVCE confirmation

• [X] Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• [X] Complete example — the example is self-contained, including all data and the text of any traceback.
• [X] Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• [X] New issue — a search of GitHub Issues suggests this is not a duplicate.
• [X] Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

No response

Anything else we need to know?

No response

Environment

INSTALLED VERSIONS ------------------ commit: None python: 3.11.0 | packaged by conda-forge | (main, Jan 14 2023, 12:26:40) [Clang 14.0.6 ] python-bits: 64 OS: Darwin OS-release: 22.6.0 machine: arm64 processor: arm byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: ('en_US', 'UTF-8') libhdf5: 1.12.2 libnetcdf: 4.8.1 xarray: 2024.1.1 pandas: 2.2.0 numpy: 1.26.4 scipy: 1.12.0 netCDF4: 1.6.0 pydap: installed h5netcdf: 1.3.0 h5py: 3.8.0 Nio: None zarr: 2.17.0 cftime: 1.6.3 nc_time_axis: None iris: None bottleneck: 1.3.7 dask: 2024.2.0 distributed: 2024.2.0 matplotlib: 3.8.2 cartopy: 0.22.0 seaborn: None numbagg: None fsspec: 2023.12.2 cupy: None pint: 0.23 sparse: None flox: None numpy_groupies: None setuptools: 69.0.3 pip: 24.0 conda: None pytest: 8.0.0 mypy: None IPython: 8.21.0 sphinx: None